Second 'device' discovered after explosion hospitalised 29 in New York neighbourhood of Chelsea

A second suspicious device, reportedly a pressure cooker with a mobile phone and wires attached, has been discovered near a blast which rocked the New York neighbourhood of Chelsea.

Fox News and ABC America reported that the device was a pressure cooker and is being investigated by police explosives experts.

ABC America added that sources said it had a mobile phone and wires attached and was contained in a plastic bag.

It was found on West 27th street, four blocks from the Chelsea blast.

Residents were told to stay away from windows in case of another explosion.

A deliberate explosion rocked an upscale and bustling neighborhood of New York, injuring 29 people, one seriously, a week after the 15th anniversary of the 9/11 attacks.

The explosion occurred in Chelsea at around 8:30 pm outside 131 West 23rd Street, at what is usually a busy time of the weekend in an area packed with bars, restaurants and luxury apartment blocks.

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

One witnessed posted a photo to Twitter of a destroyed rubbish bin.

Mayor Bill de Blasio and the city's brand-new police commissioner said there was no known link to terror at this stage, scrambling to reassure the city's 8.4 million residents that there was no specific threat.

"There is no evidence at this point of a terror connection to this incident, this is preliminary information," de Blasio told reporters.

"There is no specific and credible threat against New York City at this point in time, from any terror organization," the mayor said.

President Barack Obama was "apprised" of the explosion and will receive updates on the situation as they become available, a White House official said.

A law enforcement source said an initial investigation suggested the explosion occurred in a dumpster but the cause was still undetermined.

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

At least three people were seen being taken away from the 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 Jersey pipe bomb

The mayor said it was not immediately apparent if the blast was connected to a pipe bomb explosion hours earlier in a trash can in New Jersey.

The blast occurred in Seaside Park during a Marine Corps charity run. It caused no injuries but forced officials to cancel the event.

There were up to four timed explosives but only one detonated, Al Della Fave, a spokesman for the Ocean County prosecutor, told CNN.

But in New York, officials were tight-lipped on the cause of the blast, saying it was still under investigation but confirmed it had not been a gas leak.

"We believe it was intentional. As soon as we're able to determine what specifically caused this explosion, we will report it to you," de Blasio said.

"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.

The city, which is America's entertainment and financial capital, already has stringent security checks in place at public buildings and routinely goes on alert following militant attacks in other US cities or in Europe.

FBI and counter-terrorism units were among those who deployed to the scene Saturday, officials confirmed.

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" and said it happened outside 131 West 23rd Street at 8:30pm.

The building has not been evacuated but an "extensive search is being conducted," O'Neill told the news conference.

Police said they were investigating a possible secondary device four blocks away from the scene of the explosion, on 27th Street.

"It was really loud, it hurt my eardrums," said Tsi Tsi Mallett, who was in a car driving along 23rd Street when the explosion took place.

"My 10-year-old boy was sat in the back seat of the car, and the explosion blew the back window out."

Ms Mallett's son was not injured.

Neha Jain, 24, who lives in the neighbourhood, 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.