- Two with life-threatening injuries

- More than 20 hurt, more than a dozen missing

New York, 6am Australian EST: At least two people were killed when two buildings collapsed in East Harlem on Wednesday, according to authorities, and a senior city official suggested that there would most likely be more fatalities.

Witnesses, some more than a mile away, reported hearing what sounded like an explosion before the buildings collapsed. Flames and smoke could be seen billowing from the street, and the force of the damage blew out windows in neighbouring buildings.

"This is a tragedy of the worst kind because there was no indication in time to save people," said Mayor Bill de Blasio at a news conference.

At least 18 people were injured, including four seriously, according to city officials. The police said that two residential buildings had collapsed.

"This is an occupied building, there were people living there, we have people missing," the senior official said. "There was a complete collapse; the fire is still going so we can't make a search. There will be fatalities."

The Fire Department said it received the first report at 9.31am EST (12.31am AEST) and was still working to determine the cause.

Elizabeth Matthews, a spokeswoman for the Consolidated Edison utility, said that a report from someone claiming to smell a heavy gas odour came in at 9.13am (12.13am AEST). The person reported smelling gas in her apartment but noted that it could have come from outside.

At 9.15am (12.15am AEST) two Con Edison crews were dispatched and they arrived just after the explosion.

The two collapsed buildings were five stories, with at least a dozen apartments between the two of them. They were about 16.7 metres tall, according to Buildings Department records.

Jennifer Salas, 20, lives in one of the collapsed buildings.

She said her husband, Jordy Salas, was in the building at the time of the collapse and was still missing.

"There's six floors in the building, each floor has one apartment," she said. "Last night it smelled like gas but then the smell vanished and we all went to sleep. We tried to find the source of the gas but since the smell left, we all went to sleep."

Through tears, she begged firefighters to find her husband.

In the immediate aftermath, witnesses described a panicked scene, with people running, unsure what was happening.

As his own building rumbled on Lexington Avenue, Michael Lewis, 48, raced to the scene, arriving before most emergency personnel.

He and a small group of neighbours surrounded a van, partly submerged by the rubble, and could see four people trapped inside, Lewis said.

"I heard some moans and groans," he said.

As the group tore through the wreckage, beside a mountain of debris on the sidewalk, emergency responders arrived and pulled Lewis and the others away.

"I'm a New Yorker," Lewis said. "You've got to help the people."

David Antar, the owner of a nearby deli, felt his building shake around 9.30am.

He ran toward the scene, joined by dozens of bystanders, he said, as smoke filled the sky. Flames shot out of the buildings, and cars that were stopped at a traffic light were covered in debris.

"The whole building is in the middle of the street," Antar said.

Waldemar Infante, 24, said that there was a church and a piano store on the street level of the buildings.

"I'm sure there was people in that building when that happened," he said.

More than 200 firefighters from 44 units responded and were still working to extinguish the fire as paramedics came to the scene. They donned surgical masks as a thick, grey smoke blanketed the neighbourhood.

Metro-North Railroad service in and out of Grand Central Terminal was suspended, officials said, with debris from the buildings thrown onto the elevated train tracks.

Commuters on a southbound Metro-North train that had just passed the area said that the train shook violently. The impact felt like the last car had been hit broadside by something large, passengers said.

Records from the buildings department indicate that the rear exterior of one of the collapsed buildings, 1646 Park Avenue, had been found in 2008 to have "several vertical cracks which is hazardous for the safety of the structure". The records do not indicate that the hazard was ever fixed.

At the Global Technology Preparatory School, at 120th Street and Lexington Avenue, a blast shook the windows and startled the students and staff.

"After that initial impact, we conducted a building-wide shelter in, to make sure we secured the exits and to make sure the children were all safe in their classrooms," said David Baiz, the principal.

The 175 middle-school students were about 45 minutes into their day when the booming collapse was heard, he said.

Next, he said, the phones in the school started ringing like mad.

"Our phones have been ringing, non-stop, from parents wanting to make sure people are safe," Baiz said. "Parents are a little rattled."

Arianna Perez worked nearby.

"The explosion was in the building behind my office," she said. "Everything was shaking. I thought it was an earthquake."

When she ran outside, the windows of her office blew out.

"So we just decided to just keep running," she said, "and just trying to get away from the office because everything was just collapsing."

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