An explosion rocked the Chelsea neighbourhood of Manhattan and injured at least 29 people on Saturday, the New York City Fire Department said, prompting emergency personnel to swarm one of the most bustling areas of the city on a cool evening.

A law enforcement source said an initial investigation suggested the explosion occurred in a dumpster but the cause was still undetermined. The head of the New York Police Department's special operations division said on Twitter that a "possible secondary device has been located" in the same general area.

Another official close to the investigation told Reuters that investigators were considering the possibility that the blast was accidentally triggered by construction explosives.

But CNN reported that law enforcement sources believe an improvised explosive device caused the blast.

President Barack Obama, who was attending a congressional dinner in Washington, "has been apprised of the explosion in New York City, the cause of which remains under investigation," a White House official said. "The president will be updated as additional information becomes available," the official added.

A U.S. official said the Joint Terrorism Task Force, an interagency group of federal, state and local officials, was responding to the blast, suggesting authorities have not ruled out the possibility of a terror attack.

The task force is also investigating a pipe bomb that exploded earlier on Saturday in a plastic trash can along the route of a charity road race in the New Jersey beach town of Seaside Park. No injuries were reported in that blast, which authorities believe was a deliberate act.

'DEAFENING' EXPLOSION

At least three people were seen being taken away from the scene of the Chelsea blast in ambulances, but the severity of their injuries was not immediately clear. A car seen driving through the area had its rear window blown out.

The blast occurred at about 8:30 p.m. (0030 GMT on Sunday) between 6th Avenue and 7th Avenue on 23rd Street, a major east-west thoroughfare in the fashionable downtown neighborhood of Chelsea, J. Peter Donald, a deputy commissioner with the New York Police Department, said in a Twitter message.

Representatives of the NYPD, the Fire Department of New York and other city agencies could not be reached immediately for further comment.

A fire department spokesman said 25 people suffered minor injuries, and none was believed to have been taken to a hospital. He said no fatalities were reported.

The explosion, described by one neighbor as "deafening," happened outside the Associated Blind Housing facility at 135 W. 23rd Street. The facility provides housing, training and other services for the blind.

Hundreds of people were seen fleeing down the block on a cool early autumn evening, as police cordoned off the area.

"It was really loud, it hurt my eardrums. My 10-year-old boy was sat in the back seat of the car, and the explosion blew the back window out," said Tsi Tsi Mallett, who was in a car driving along 23rd Street when the explosion took place. Her son was not injured.

Neha Jain, 24, who lives in the neighborhood, said she was sitting in her room watching a movie when she suddenly heard a huge boom and everything shook.

"Pictures on my wall fell, the window curtain came flying as if there was a big gush of wind. Then we could smell smoke. Went downstairs to see what happened and firemen immediately told us to go back."

New York City Police issued a bulletin advising motorists in the area that they should "expect extensive traffic delays and emergency personnel in the area of 23rd Street and 7th Avenue" due to police activity there and asking the public to avoid the area. – Reuters