NEW YORK (REUTERS/AFP) - A loud explosion shook the Chelsea neighbourhood of Manhattan on Saturday (Sept 17) night, prompting New York City police and fire department personnel to swarm the area, as the city's mayor said it appeared to be "intentional".

"Explosion happened at roughly 8.30pm on 23rd street between 6th and 7th avenues," in Manhattan, police spokesman J. Peter Donald tweeted. "Several injured transported to area hospitals."

Police combing the area around the scene found a pressure cooker nearby connected to a cellphone with wires attached, CNN reported, citing law enforcement sources. The possible secondary explosive device was four blocks away from the scene of the explosion, on 27th Street. A piece of paper with writing on it was found nearby, according to CNN's account.

Residents living nearby were advised to stay away from windows facing the street as a precaution, and the item was later safely moved to a police firing range for further examination. As of Sunday morning (US time), police were still seeking to determine whether the item was an explosive and had not detonated it, said New York police Lieutenant Thomas Antonetti.

Pressure cookers packed with explosives and detonated with timing devices were used by two Massachusetts brothers in the 2013 Boston Marathon bombing that killed three people and wounded more than 260.

New York Mayor Bill de Blasio said at a news conference that the explosion appears to have been an "intentional act", but there is no "evidence at this point of a terror connection". "It's something we will be investigating very carefully, but there is no evidence at this point of a terror connection," he added.

The mayor said it was not immediately apparent if the blast was connected to an explosion in a trash can in New Jersey earlier on Saturday.

"I want to assure all New Yorkers that the NYPD and all other agencies are at full alert and that our anti-terror capacity in particular is at full alert," he said.

Twenty-nine injuries were reported, with none of them life threatening, the city's Fire Commissioner Daniel Nigro said at the news conference. But one injured person is in serious condition, officials added.

Of the wounded, 24 have been taken to hospitals with various degrees of scrapes and abrasions from glass and metal, said Mr Nigro.

A law enforcement source said an initial investigation suggested the explosion occurred in a dumpster but the cause was still undetermined. 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. It was not caused by gas, the police said.

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

Mr James O'Neill, who took over as New York police chief only on Saturday after veteran commissioner Bill Bratton resigned, described the explosion as "large". The building has not been evacuated but an "extensive search is being conducted," he told the news conference.

The explosion, described by one neighbour as "deafening", happened outside the 14-storey Associated Blind Housing facility at 135 W. 23rd Street. The facility provides housing, training and other services for the blind.

The fire department was at the site of the blast, an area of many restaurants, bars, theatres and apartments. Police blocked off the area around the scene. The New York Post reported that two buildings east of the blast site were being evacuated.

At least three people were seen being taken away from the apparent scene of the 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.

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.

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.

Republican nominee Donald Trump jumped the gun on the news reports by saying in Colorado Springs that "a bomb went off in New York". "We better get very tough, folks, we better get very, very tough," he said.

His Democratic rival Hillary Clinton immediately took swipe at her opponent. "I think it's always wiser to wait until you have information before making conclusions," she said.

23rd Street is a major east-west thoroughfare in the fashionable downtown neighbourhood of Chelsea. Chelsea is one of the most fashionable districts of Manhattan, packed with bars, restaurants and luxury apartments that would be crowded in the weekend.

An AFP journalist at the scene said police had sealed off the area and shut down the closest subway station. Police helicopters circled overhead. A photograph shared by New York's local NY1 television station showed shattered glass in a doorway, apparently caused by the blast.

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 Ms Tsi Tsi Mallett, who was in a car driving along 23rd Street when the explosion took place. Her son was not injured.

Ms 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."

The blast comes hours after a pipe bomb exploded in a garbage can in Seaside Park, New Jersey during a Marine Corps charity run.

It caused no injuries but forced officials to cancel the event.

The New Jersey explosion was set for the moment that thousands of runners would be passing the can - but there were no casualties because the race was delayed, said Mr Al Della Fave, a spokesman for the Ocean County prosecutor.

There were up to four timed explosives but only one detonated, Mr Fave told CNN.

New York, which last weekend marked the 15th anniversary of the 9/11 attacks, is the largest city in the United States with a population of 8.4 million people.