An explosion in the Chelsea neighbourhood of New York City appears to have been an "intentional act," but there is no "evidence at this point of a terror connection," Mayor Bill de Blasio said at a news conference.

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

The blast caused at least 29 injuries on Saturday, one serious but none life-threatening, the New York City Fire Department said.

LIVE on <a href="https://twitter.com/hashtag/Periscope?src=hash">#Periscope</a>: NYPD live update on NYC explosion <a href="https://t.co/ugjzliOWUD">https://t.co/ugjzliOWUD</a> —@cbcsteve

The exact cause of the explosion is not known.

The blast occurred at about 8:30 p.m. ET between 6th Avenue and 7th Avenue on 23rd Street, a major east-west thoroughfare in the fashionable downtown neighbourhood of Chelsea.

"Now, I want to be clear whatever the cause, whatever the intention here, New Yorkers will not be intimidated. We are not going to let anyone change who we are or how we go about our lives," de Blasio said.

2nd site

Police Commissioner James O'Neill says officers are investigating a possible second explosive device a few blocks away from the explosion at about 8:30 p.m. on West 23rd Street.

CNN and The Associated Press cited anonymous police sources saying the second device appeared to be a pressure cooker attached to wiring and a cellphone.

NYPD issued a statement early shortly after midnight asking residents on West 27th Street to stay away from their windows until police "clear the area of a suspicious package."

Please be advised of this update regarding the incident in Chelsea, NYC <a href="https://t.co/Ux5dfBsj72">pic.twitter.com/Ux5dfBsj72</a> —@NYPDnews

The explosion, described by one neighbour 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.

A number of New York City subway routes were affected.

Police and firefighters work near the scene of an apparent explosion in Manhattan's Chelsea neighbourhood. At least 29 people were injured, but all are expected to survive. (Andres Kudacki/Associated Press)

"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.

'We could smell smoke'

Her 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."

Onlookers stand behind a police cordon near the site of Saturday night's explosion in New York City. (Umar Abbasi/Reuters)

The White House said President Barack Obama has been apprised of the explosion in New York City and will be updated as additional information becomes available.

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.

Democratic presidential candidate Hillary Clinton says she has been briefed on the explosion. She said the nation needs to support its first responders and "pray for the victims."

After stepping off his plane during a rally at an airport hangar in Colorado Springs, Colo., Republican candidate Donald Trump said: "Boy we are living in a time — we better get very tough, folks. We better get very, very tough."

Not connected to N.J. pipe bomb

Earlier Saturday, a pipe bomb exploded in Seaside Park, N.J., shortly before thousands of runners participated in a charity 5K race to benefit Marines and sailors. No injuries were reported in what authorities believe was a deliberate attack.

There is no indication the two explosions are connected, de Blasio said.