KOLKATA, India — Using saws, small cranes and bare hands, rescuers searched for survivors Friday under the crumbled concrete and twisted steel from an overpass that collapsed onto a crowded Kolkata neighborhood, killing at least 23 people and injuring more than 80.

With more than half the debris cleared by Friday morning, 67 people have been pulled out alive, Kolkata police Sgt. P. Chakraborty said. But more people were still feared trapped. It was not clear how many are missing.

Smashed yellow taxis, a crushed truck, destroyed rickshaws and the bloody legs of trapped people jutted from the fallen girders and concrete. Building and other construction collapses are common in India, where regulations are poorly enforced and companies often use substandard materials.

The partially constructed overpass spanned nearly the width of the street and was designed to ease traffic through the densely crowded Bara Bazaar neighborhood in the capital of the east Indian state of West Bengal. The steel girders had already been fixed, and on Thursday the concrete was poured into the framework.

Within hours, as the concrete was drying, about 100 meters of the overpass fell, while other sections remained standing.

"I heard an explosion, a solid one," said resident Rabindra Kumar Gupta, who had been home eating lunch when the overpass crashed down around 12:30 p.m. Thursday. "My apartment shook. The whole building shook. When I looked outside, there was a lot of smoke."

Another resident, Yogesh Sharma, described a "huge crashing sound" when the overpass came down as he was been sitting at a roadside tea stand with friends.

"I left my cup of tea and ran," said Sharma. "I was crying at the spot."

Crowds waited anxiously near the rescue area to see if neighbors and friends had survived. The intersection had been a place where street vendors and service workers regularly plied their trades.

"There used to be a tailor who sat here on this corner. We wonder about him. A cigarettes and tobacco vendor — we knew everyone who used to stay around this crossing," resident Pankaj Jhunjhunwala said. "Until this rubbish is removed, we can't say for sure where they are or how this happened."

Police said 39 of the more than 80 people taken to hospitals were still being treated Friday morning. At least 23 people were killed.

With army troops and personnel from the National Disaster Response Force joining the effort, police said they expected the rescue and cleanup to be completed on Friday.

A damaged three-wheeled vehicle is taken out from the rubble of a collapsed overpass in Kolkata, India, Friday, April 1, 2016. Photo: Bikas Das / Associated Press

Workers in yellow hard-hats operated huge cranes, bulldozers and other equipment through the night to clear the rubble and pry apart the concrete slabs. They also used cutting torches to break up metal beams.

The operation was a "very, very challenging task," said O.P. Singh, chief of the disaster response force. Rescuers also used dogs and special cameras to find people who were trapped, he said.

"The area was very, very crowded. Motorized rickshaws, taxis … there was a lot of traffic," one witness told NDTV television.

Prime Minister Narendra Modi, who was in Washington at the time of the collapse, called Chief Minister Mamata Banerjee, the top elected official of West Bengal state, to express grief at the tragedy and pledge federal support.

He said he was "shocked and saddened," according to a message on his Twitter account. "My thoughts are with the families of those who lost their lives in Kolkata. May the injured recover at the earliest."

Banerjee, whose has been campaigning for re-election this month, told reporters that a private builder had missed several deadlines for completing the construction.

General view shows a partially collapsed overpass in Kolkata, India, Friday, April 1, 2016. Photo: Bikas Das / Associated Press

The contract for the overpass was signed in 2007 and it was expected to be completed in two years. Banerjee accused the previous Communist government in West Bengal of not adhering to building regulations.

"We completed nearly 70 percent of the construction work without any mishap," said K.P Rao, a top official of IVRCL Infrastructure company, which was building the overpass. "We have to go into the details to find out whether the collapse was due to any technical or quality issue."

Police have sealed the Kolkata office of the Hyderabad-based construction firm involved in building the overpass. It is also investigating the firm's officials for alleged culpable homicide, punishable with life imprisonment, and criminal breach of trust, which carries a prison sentence of up to seven years.

Story: Rishi Lekhi and Rishabh R. Jain / Associated Press

WASHINGTON — In the face of mounting threats from North Korea, President Barack Obama on Thursday urged closer security ties among its chief allies in Asia and increased cooperation with strategic rival China to discourage Pyongyang from further advances in nuclear weapons.

As world leaders gathered for a nuclear security summit, Obama first met with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye. Together, they warned North Korea would face even tougher sanctions and more isolation if provokes again with nuclear and missile tests.

Then Obama met Chinese President Xi Jinping and both called for North Korea to give up its nuclear weapons. China also agreed to implement in full the latest economic restrictions imposed by the U.N. Security Council against Pyongyang.

More than 50 governments and international organizations are attending the two-day summit on preventing nuclear terrorism — the last in a series of global meetings Obama has championed on the issue. The risk posed by the Islamic State group tops this year's agenda but concerns about North Korea are also commanding focus.

"Of great importance to both of us is North Korea's pursuit of nuclear weapons, which threatens the security and stability of the region. President Xi and I are both committed to the denuclearization of the Korean Peninsula," Obama said at the start of his meeting with Xi.

"China and the U.S. have a responsibility to work together," Xi said in his comments made to reporters through an interpreter. As for their "disputes and disagreements," the Chinese leader said the two sides could "seek active solutions through dialogue and consultation."

North Korea's fourth nuclear test in January, followed by a space a launch in February, have heralded more convergence among often-fractious powers in East Asia — at least on the need to press the government of Kim Jong Un toward disarming.

Japan and South Korea have persuasive reasons to get along. They both host U.S. forces and are both in range of North Korean missiles. But their relations have been plagued by historical differences that date back to Japan's colonial occupation of Korea in the first half of the 20th century and its military's use of sex slaves during World War II.

But those tensions have eased some. Abe said North Korea nuclear and missile capability is a "direct and grave threat" to them all.

"Should it choose to undertake yet another provocation, it is certain to find itself facing even tougher sanctions and isolation," Park said of Pyongyang.

South Korean President Park Geun-hye, left, shakes hands with Japanese Prime Minister Shinzo Abe as President Barack Obama watches after their meeting at the Nuclear Security Summit in Washington, on Thursday, March 31, 2016. (AP Photo/Jacquelyn Martin)

Young leader Kim Jong Un has also alienated the North's traditional benefactor and main trading partner, China. The U.S. has long urged Beijing to take a more forceful role in pressing North Korea, and Chinese Vice Foreign Minister Zheng Zeguang said after the Obama-Xi meeting that the two sides agreed the new U.N. resolution "should be implemented in full and in its entirety."

The U.S. and China also released joint statements vowing robust collaboration to improve nuclear security and to implement a global climate change deal, and reported progress on the issue of cyber security.

But they were at stark odds in other areas.

According to Zheng, Xi told Obama that China was "firmly opposed" to the U.S. deploying a new missile defense system in South Korea, saying it was against China's national security interests and would the effect the strategic balance in the region.

The U.S. and Seoul are considering that deployment to counter the threat from the North. China contends the system would also give the U.S. radar coverage over Chinese territory. Russia opposes it as well.

Washington has also opposed China's move to build artificial islands and military facilities in the disputed South China Sea. Japan and South Korea are similiarly concerned about China's military build-up and assertive actions in the region's disputed waters.

Tensions appear set to intensify with an upcoming ruling from an international tribunal that could challenge the legal basis of some of Beijing's sweeping territorial claims. The U.S. has supported the right of its ally, the Philippines, to submit the case and says the ruling should be binding on both parties, although China has boycotted the proceedings and says it will ignore it.

Xi told Obama that the South China Sea islands — claimed by several other Asian governments — have been China's territory since ancient times and it has the right to defend its territorial sovereignty and maritime rights, Zhang said.

Obama also met Thursday with French President Francois Hollande, amid steep concerns about terrorism in Europe following Islamic State-linked attacks in Paris and Brussels. The nuclear security summit continues on Friday with a special session focused on preventing IS and other extremists from obtaining nuclear materials and attacking urban areas.

On Thursday, the U.S. said a strengthened nuclear security agreement among nations was finally set to take force following ratification by a critical mass of countries. The stricter rules include new criminal penalties for smuggling nuclear material and expanded requirements for securing materials and nuclear facilities worldwide, and are intended to reduce the likelihood of terrorists getting their hands on ingredients for a bomb.

The United States says it's making progress in reducing its stockpiles of highly enriched uranium. The White House says it's declassifying and publicly releasing a national inventory of highly enriched uranium for the first time since 1996. As of late 2013, the U.S. had 586 metric tons of highly enriched uranium. That's a drop from the 741 metric tons the U.S. had in 1996.

Fissile materials like highly enriched uranium and separated plutonium can be used to make nuclear bombs.

Story: Matthew Pennington and Josh Lederman / Associated Press