An apparent gas explosion has levelled two buildings and sparked a fire in New York City, killing at least six people and injuring more than 50 others.

Two five-storey buildings were reduced to rubble in the blast near Park Avenue and 116th Street in East Harlem.

New York Mayor Bill de Blasio said more than a dozen people were missing, and that a search effort would begin after firefighters put out the blaze.

The mayor emphasised that some of those considered missing could be in another location and authorities are attempting to make contact with all concerned.

“This will take quite a bit of time to address,” he said.

New York’s Hunter College identified one of the victims as Griselde Camacho, a security officer who had worked for the college since 2008.

Carmen Tanco, 67, a dental hygienist, was also killed in the blast.

The identities of the othervictims have not been confirmed.

The fire department said two people suffered life-threatening injuries, while five more suffered serious injuries.

The remaining injuries have been classified as minor.

Witnesses reported hearing a large explosion around 9.30am, after a local resident reported smelling natural gas.

A spokesman for Con Edison said the utility company dispatched two crews, but the blast occurred before they arrived.

The utility said the person who reported the gas smell said he thought the odour could be coming from outside.

Mr de Blasio said crews from Con Edison and other utilities were working to shut down services to the area.

Aerial footage showed a large plume of smoke billowing over the Upper Manhattan skyline and debris scattered across the pavement and vehicles parked on adjacent streets.

Waldemar Infante, who was working in the basement of the neighbouring building when the explosion took place, said it felt like an earthquake.

“There were glass shards everywhere on the ground and all the stores had their windows blown out,” he said.

New York Correspondent Hannah Thomas-Peter described an “acrid smell” at the scene and said people were wearing masks due to lingering smoke.

Witnesses said the blast was so powerful it knocked groceries off the shelves of nearby stores.

Eoin Hayes, 26, said the explosion shook his entire apartment building.

He said he ran to the window and saw flames consuming one building and smoke rising into the air.

“I was in my bedroom and the explosion went off, it kind of shook the whole building,” Mr Hayes said. “You could feel the vibrations going through the building.”

The FDNY says it has over 200 personnel at the scene.

Metro-North Railroad has suspended all commuter train service in the area. The train line runs along the scene of the collapse, and television footage showed some debris on the tracks.

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