At least 29 people injured in "intentional" explosion in New York

One person was described as 'seriously injured' following the blast in the Chelsea district

At least 29 people injured in "intentional" explosion in New York

Police and firefighters work near the scene of an apparent explosion in Manhattan's Chelsea neighborhood, in New York. Image: Andres Kudacki / AP/Press Association Images

Updated: 18.35

An explosion that ripped through a busy Manhattan neighbourhood injuring at least 29 people was an "intentional act", according to the authorities.

Nearly 1,000 extra state police and national guard troops will be deployed in New York as authorities vow to catch those behind a bomb attack in Manhattan.

Police combing the area with sniffer dogs following the blast in the fashionable Chelsea district, also discovered a second suspected device nearby.

The pressure cooker connected to a mobile phone with wires attached had been placed in a plastic bag inside a bin.

A piece of paper with writing on it was also found next to it, according to CNN.

It was later removed by bomb disposal teams using a remote-controlled robot.

The security alert came just hours after a pipe bomb blast at a New Jersey charity run in aid of US Marines and sailors.

New York, which only recently marked the anniversary of 9/11, is to host world leaders this week at a meeting of the United Nations.

Initial investigations suggest the Chelsea explosion, described as "deafening", occurred inside a large bin or a construction toolbox.

Pictures from the scene showed a twisted and crumpled black metal box.

Witnesses said the blast at about 8.30pm local time, blew out the windows of buildings and sent debris flying across the area.

Rudy Alcide, a nightclub bouncer, said: "It was an extremely loud noise, everything was shaking, the windows were shaking, it was crazy.

"It was extremely loud, almost like thunder, but louder."

Kimberly Gantz tweeted: "Omg that explosion knocked us out of our seats on my friends [sic] terrace in #Chelsea #NYC."

Following the explosion, Facebook activated its safety check service that lets users notify friends and family that they are safe.

It was the first time it had been used in New York.

The city's fire commissioner Daniel Nigro said of those hurt, 24 had been taken to hospital, including one person he described as seriously injured.

Ambulances carrying the wounded were checked by armed police on arrival.

New York Mayor Bill De Blasio said early indications were that the explosion was "an intentional act".

But he added: "There is no evidence at this point of a terror connection.

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

Speaking after a separate attack in Minnesota, Chief William Blair Anderson of the St. Cloud police force stated that "Right now we don't have any evidence to suggest that they were connected."

Experts are keen to know what the difference is between the two, and former Mayor Rudy Giuliani thinks it's too early to call:

Similarly, anti-terrorism consultant and bomb expert Chris Hunter says this view is bizarre:

However, a Joint Terrorism Task Force, made up of different law enforcement agencies, has been called in to investigate, indicating the authorities have not ruled out the possibility of a terror link.

Surveillance video from businesses in the area will be examined by investigators hunting for clues to the explosion.

President Barack Obama has been "apprised" of the situation, according to a White House official.

The two presidential candidates have also responded to the explosion.

Democratic nominee Hillary Clinton said the nation needed to support its emergency workers and "pray for the victims".

She added: "We have to let this investigation unfold."

However, her Republican rival Donald Trump was more forthright saying: "I must tell you that just before I got off the plane a bomb went off in New York and nobody knows what's going on.

"But boy we are living in a time - we better get very tough, folks.

"We better get very, very tough. It's a terrible thing that's going on in our world, in our country and we are going to get tough and smart and vigilant."