New York City firefighters stand near the site of an explosion in the Chelsea neighborhood of Manhattan, New York, US, September 17, 2016. [Photo/Agencies]

Donald Trump, 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.

Apir Chaudhury, a sales associate working on a night shift at a 7-11 convenience store on 5th avenue, said that he didn't hear the explosion, but around 9 p.m., police closed up the roads and started to check everything. The young man looked undisturbed by the incident at all, saying "Now I just wanna go home and sleep".

After living in this city for six years, you don't get scared by such things", a female cashier in the store added.

Officer Bressack with the NYPD's Strategic Response Group told Xinhua that he and his colleagues were called in for duty shortly after the blast occurred, and rushed all the way from Queensboro to the scene.

There could be all kinds of causes for the explosion, and people have to wait for the investigation results. "I have seen so many of this, and I can only say this was not the first and won't be the last," said the police veteran who had served for 22 years.

New York City Police Department issued an updated statement, saying "As a precautionary measure, we are asking residents who live on West 27th Street between 6th and 7th Avenues in Manhattan to stay away from windows facing 27th Street until we clear the area of a suspicious package. This is a precautionary measure only, we are not evacuating the area".