A further eight people have been injured and have been taken to hospital, according to the New York City Fire Department.

Fire personnel found multiple people shot and undetonated devices. after responding to reports of smoke at around 8:30am at the 36th Street station in Sunset Park.

Photos and video from the scene showed people tending to bloodied passengers lying on the floor of the station.

US President Joe Biden has been briefed on the developments and senior staff are in contact with the city's mayor, Eric Adams, White House press secretary, Jen Psaki said.

Fire and police officials were investigating reports that there had been an explosion, but the city's police department said there are no active explosives at this time.

'My subway door opened into calamity'

A man was spotted throwing a device in the Brooklyn subway station and then opened fire, law enforcement sources told NBC New York.

It is understood that preliminary information indicates a suspect was dressed in construction gear.

Eyewitness, Sam Carcamo, told radio station 1010 WIN he saw a gigantic billow of smoke pouring out of the train once the door opened.

"My subway door opened into calamity. It was smoke and blood and people screaming", he said.

A bystander video shows people lying on the subway platform amid what appeared to be small puddles of blood, as a loudspeaker announcement told everyone on the smoke-hazy platform to get on a train.

Outside the station, a police officer yelled, "Let's go! Get out of the way".

The incident happened on a subway line that runs through south Brooklyn in a neighborhood about a 15-minute train ride to Manhattan.

Local schools, including Sunset Park High School across the street, were locked down.

Witnesses have been urged to provide information whilst the public has been advised to stay away from the area.

Video footage also showed a large law enforcement presence around the subway station, including heavily armed officers and dozens of police cruisers and emergency vehicles.

It was not immediately clear whether officers had already been inside the station when the shootings occurred.