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]

At least three people were seen being taken away from the apparent scene of the blast in ambulances, but the severity of their injuries was not immediately clear. A car seen driving through the area had its rear window blown out.

Police and fire representatives said they were investigating reports of an explosion.

New York City Police issued a bulletin advising motorists in the area that they should "expect extensive traffic delays and emergency personnel in the area of 23rd Street and 7th Avenue" due to police activity there and asking the public to avoid the area.

The blast, according to a Reuters witness, occurred on 23rd Street, a major east-west thoroughfare in the fashionable downtown neighborhood of Chelsea.