Police at the scene of an explosion in Manhattan's Chelsea neighbourhood in New York (AP)

A police officer escorts an injured man away from the scene of the blast on West 23rd Street in New York (AP)

The scene of an explosion in Manhattan's Chelsea neighbourhood in New York (AP)

Investigators at the scene of Saturday's explosion in Manhattan's Chelsea neighbourhood in New York (AP)

An explosion that rocked a crowded Manhattan neighbourhood, injuring 29 people, does not appear to be linked to international terrorism, New York governor Andrew Cuomo has said.

Mr Cuomo said 1,000 additional law enforcement officers were being deployed after the Saturday night blast in Chelsea, a primarily residential neighbourhood on Manhattan's west side that is known for its art galleries and large gay population.

He encouraged New Yorkers to go about their day as usual.

"We're not going to let them win," Mr Cuomo said at the scene. "We're not going to let them instil fear."

The Democratic governor said the preliminary investigation did not appear to show a link to international terror, and he noted that no terror group had taken credit for it.

Authorities said the Manhattan blast did not appear to be connected to a pipe bomb explosion earlier on Saturday in New Jersey that forced the cancellation of a charity run. He noted that the bombs included different materials.

A law enforcement official said a second device that officers investigated four streets from the scene appeared to be a pressure cooker attached to wiring and a mobile phone. The official said the device was found inside a plastic bag on West 27th Street. The device was removed with a robot and taken to a department firing range in the Bronx.

The law enforcement official also said that the explosion appeared to have come from a construction toolbox in front of a building. Photos from the scene show a twisted and crumpled black metal box.

The blast happened on West 23rd Street, in front of a residence for the blind, near a major thoroughfare with many restaurants and a Trader Joe's supermarket. Witnesses said the explosion at about 8.30pm local time blew out the windows of businesses and scattered debris.

Mr Cuomo said on Sunday that all the injured who were taken to hospital after the blast had been released. He said most had been hit with glass or debris.

Some New York City subway routes were affected by the explosion, which rattled some New Yorkers and visitors on the heels of the 15th anniversary of the 9/11 terror attacks.

Chris Gonzalez, visiting from Dallas, was having dinner with friends at a restaurant in the area.

"We felt it. We heard it," Mr Gonzalez said. "It wasn't like jolting or anything. Everyone just went quiet."

Rudy Alcide, a bouncer at Vanity Nightclub at 21st Street and 6th Avenue, said at first he thought something large had fallen.

"It was an extremely loud noise. Everything was shaking, the windows were shaking," he said. "It was extremely loud, almost like thunder but louder."

The FBI and Homeland Security officials, along with the Bureau of Alcohol, Tobacco, Firearms and Explosives arson and explosive task force, were at the scene.

Mr Cuomo toured the site on Sunday.

The White House said President Barack Obama was made aware of the explosion.

Democratic presidential nominee Hillary Clinton said the nation needs to support its emergency services staff and "pray for the victims".

"We have to let this investigation unfold," she said.

Republican presidential nominee Donald Trump moved ahead of New York City officials when he declared a "bomb went off" before officials had released details. He made the announcement minutes after stepping off his plane in Colorado Springs, Colorado.

"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," Mr Trump said.

He continued: "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."

The Manhattan blast came hours after a pipe bomb exploded in Seaside Park, New Jersey, shortly before thousands of runners were due to participate in a charity 5km race to benefit marines and sailors. The run was cancelled and no injuries were reported.

Also on Saturday, at least eight people were injured at a shopping centre in St Cloud, Minnesota, in a stabbing attack. The suspect was shot dead by an off-duty police officer. The police chief said the suspect asked at least one victim if he or she was Muslim.

The investigation into the Manhattan explosion came as world leaders descend on the United Nations for a meeting on Monday to address the refugee crisis and the Syrian conflict. The blast site is about two miles away.

AP

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