A caller to US emergency services warned of more explosions right after the bomb that went off in New York injuring 29 people.

The New York Post has reported that police sources had advised that someone called 911 claiming to be standing close to where the bomb went off.

“I’m looking at the explosion down the block. There will be more,” the man reportedly told emergency services.

It is understood the blast was the result of a home-made, pressure-cooker bomb similar to an unexploded device found later on four blocks away from the Chelsea blast.

The Post also reported that a hand-written letter, part of which was in Arabic, was found inside a plastic bag containing the second device.

However, The Post also reported that police were investigating a Tumblr post which claimed responsibility for the first bomb, as a protest against discrimination of the LGBTI community.

Witnesses claim to have seen man carrying suitcase

Cops were interviewing two men who claim to have spotted a potential person of interest in the case, sources said.

The witnesses were dining at the Krush bar and grill on 32nd Street about a half-hour before the 8.30pm Saturday explosion when they saw an Arab-looking man who was carrying a suitcase and appeared out of place, sources said.

New York authorities moved quickly to calm the city’s population after an explosion in the Manhattan district of Chelsea left 29 people injured.

New York Mayor Bill de Blasio called the blast on Saturday an “intentional act”, but said there was no terrorist connection.

“Tonight, New York City experienced a very bad incident,” de Blasio told a news conference near the scene in Chelsea. “We have no credible and specific threat at this moment.”

De Blasio said the blast was “an intentional act” and tried to calm any fears among nervous New Yorkers, saying the explosion had no terrorist connection and wasn’t related to a pipe bomb explosion earlier on Saturday in New Jersey at a charity run.

He promised to ramp up the city’s police presence, which has already been increased for United Nations General Assembly week.

A law enforcement official told The Associated Press that a second device that officers were investigating four blocks from the scene appeared to be a pressure cooker attached to wiring and a mobile phone. 

A possible secondary device has been located @ 27th Street, 6th – 7th Avenues. #NYPD nvestigations are ongoing. Avoid the area @NYPD13Pct

— NYPD Special Ops (@NYPDSpecialops) September 18, 2016

The source, speaking on condition of anonymity because the person was not authorised to speak about an ongoing investigation, said the device was found inside a plastic bag on West 27th Street.

None of the people injured in the Chelsea blast suffered life-threatening wounds, New York City fire commissioner Daniel Nigro said.

A police spokesman said several people were taken to hospitals with injuries. One of the injured suffered a puncture wound that was considered serious. Officials said the other injuries were minor, described as scrapes and bruises.

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

The blast prompted New York City police and fire department personnel to swarm the area.

A car seen driving through the area had its rear window blown out.

Anyone else catch that huge explosion in Chelsea NYC? Crazy world we live in. 😱😱 #NYC #Chelsea pic.twitter.com/QBK1ZR2l2B

— Raymond Strazdas (@raystrazdas) September 18, 2016

New York City Police issued a bulletin advising motorists that they should “expect extensive traffic delays and emergency personnel in the area” due to police activity.

They asked the public to avoid the area.

The blast occurred at about 8.30pm (local time) between 6th Avenue and 7th Avenue on 23rd Street, a major east-west thoroughfare in the fashionable downtown neighbourhood, J Peter Donald, a deputy commissioner with the New York Police Department, said on Twitter.

“Several injured transported to area hospitals,” he wrote.

https://twitter.com/voicehalf/status/777306679029424128

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

The explosion, described by one neighbour as “deafening”, happened outside the Associated Blind Housing facility, which provides housing, training and other services for the blind.

https://twitter.com/regated/status/777346138135490560

New Jersey pipe bomb

About 135 kilometres south of Manhattan, a pipe bomb exploded in New Jersey shortly before thousands of runners were due to participate in a charity race to benefit Marines and sailors.

No injuries were reported in the blast in Seaside Park about 9.30am on Saturday, said Al Della Fave, spokesman for the Ocean County Prosecutor’s office.

He also said no surrounding structures suffered any apparent damage.

The third annual Semper Five run was cancelled and the immediate area put on lockdown.

Authorities immediately cleared the Seaside Park boardwalk and beach area after the explosion. Photo: Getty

The pipe bomb was in a plastic garbage can when it exploded, Della Fave said. Authorities immediately cleared the boardwalk and beach area and bomb-sniffing dogs were used to conduct a search.

Beaches in Jersey Shore town reopen even as investigation into pipe bomb continues https://t.co/Kma6HuctqT pic.twitter.com/4omtbNvkJp

— ABC News (@ABC) September 18, 2016

Authorities said there was a report of at least one other explosive device that a New Jersey State Police bomb team would “make safe” but further details were not disclosed.

Officials said runners had been scheduled to pass by the area about the time the blast occurred. But the start of the race had been delayed after an unattended backpack was found and the area was mostly empty.

Meanwhile Australian Prime Minister Malcolm Turnbull, in New York for the UN meeting, said in the wake of both the Chelsea blast and a mass stabbing in Minnesota, that people should defy those wanting to do harm by going about their lives.

Turnbull told reporters in Central Park it was important to learn from the attacks but also to not be intimidated.