NEW YORK - An explosion rocked one of the most fashionable neighbourhoods of New York on Saturday night, injuring 29 people, one seriously.

Mayor Bill de Blasio indicated the blast was not accidental, even if there was no known link to terrorism. "We believe it was intentional. As soon as we're able to determine what specifically caused this explosion, we will report it," de Blasio said.

The mayor made the remarks at a news conference, saying the real cause of the explosion is still under investigation.

"Early indications are that this was an intentional act," the mayor said, adding "There is no specific and credible threat against New York City at this point in time from any terror organisation."

Law Enforcement sources have also reportedly said they found a device that appeared to be pressure cooker near the explosion site.

The incident occurred just three days before the start of a high-level debate in the 71st Session of the UN General Assembly at the UN Headquarters in New York. The host city of the world body is already on high alert to guarantee the security of visiting heads of state or government.

Donald Trump, the Republican presidential nominee, gave a speech in Colorado about 30 minutes after the blast. "Just before I got off the plane, a bomb went off in New York and nobody knows exactly what's going on," Trump said. "We've got to get very tough," Trump said. "It's a terrible thing what's going on in our world, what's going on in our country, but we are going to get tough and smart and vigilant and we are going to end it."

Meanwhile, Democratic presidential nominee Hillary Clinton said she has been briefed "about the bombings in New York and New Jersey and the attack in Minnesota. "She said the nation needs to support its first responders and "pray for the victims."

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

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

James O'Neill, who took over as New York police chief only on Saturday after his predecessor resigned, described the Manhattan explosion as "large" and said it happened outside 131 West 23rd Street at 8:30pm.

He added that the building has not been evacuated and that an "extensive search" was being conducted.

The blast occurred in Chelsea - an area packed with bars, restaurants and luxury apartment blocks - at a typically bustling time of the weekend.

AFP reporters saw a massive police presence at the crime scene, where they were joined by FBI and counter-terrorism squads, as police helicopters circled overhead and sirens wailed.

Of the 29 people who sustained injuries, 24 were taken to hospital with various degrees of scrapes and abrasions from glass and metal, said Fire Department commissioner Daniel Nigro.

Witnesses living three blocks away told AFP they heard a large boom from their fifth floor apartment, followed by quiet, then the sound of sirens.

A photograph shared by New York's local NY1 television station showed shattered glass in a doorway, apparently caused by the blast.

Public 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.