A gunman in a construction vest donned a gas mask, set off a smoke canister on a rush-hour subway train and shot at least 10 people, authorities in New York City have said.

Police were scouring the city for the gunman and found a rental truck possibly connected to the violence.

A scene of horror unfolded as frightened commuters ran from the train as others limped out of it.

At least one rider collapsed on the platform.

“My subway door opened into calamity. It was smoke and blood and people screaming,” witness Sam Carcamo told radio station 1010 WINS.

Smoke poured out of the train car as the door opened, he added.

Five people were in a critical condition but expected to survive.

At least 29 in all were treated in hospital for gunshot wounds, smoke inhalation and other conditions.

The gunfire erupted on a train that pulled into a station in the Sunset Park neighbourhood, about a 15-minute ride from Manhattan and predominantly home to Hispanic and Asian communities.

Police commissioner Keechant Sewell said the attack was not being investigated as terrorism, but that she was “not ruling out anything”.

The gunman’s motive was unknown.

Authorities gave officers a photo they believed to depict the gunman, and the Arizona licence plate number of a U-Haul truck to look out for, two law enforcement officials said.

By early evening, police found a matching, unoccupied U-Haul van in Brooklyn, one of the officials said.

Police were closing off a street about four miles from the shooting scene and clearing nearby businesses while awaiting a bomb squad and the highly specialised emergency services unit.

Investigators recovered a handgun at the scene, along with multiple smoke devices and other items they are analysing, said the officials.

They said the suspect is believed to have had at least two extended magazines.

Investigators believe the weapon jammed, preventing the suspect from continuing to fire, the officials said.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has completed an urgent trace to identify the gun’s manufacturer, seller and initial owner.

The attack unnerved a city on guard amid a rise in gun violence and the ever-present threat of terrorism.

It left some New Yorkers jittery about riding the nation’s busiest subway system and prompted officials to increase policing at transportation hubs from Philadelphia to Connecticut.

One rider’s video, shot through a closed door between subway cars, shows a person in a hooded sweatshirt raising an arm and pointing at something – possibly the door to a conductor’s booth – as five bangs sound.

In another video, smoke and people pour out of a subway car and wails erupt as passengers run for an exit while a few others limp off the train.

One falls to the platform, and a person shouts: “Someone call 911!”

Other video and photos from the scene show people tending to bloodied passengers lying on the platform, some amid what appear to be small puddles of blood, and another person on the floor of a subway car.

Juliana Fonda, a broadcast engineer at WNYC-FM, told its news site Gothamist she was riding the train when passengers from the car behind hers started banging on the door between them.

“There was a lot of loud pops, and there was smoke in the other car,” she said.

“And people were trying to get in and they couldn’t, they were pounding on the door to get into our car.”

As police searched for the gunman, governor Kathy Hochul warned New Yorkers to be vigilant.

“This individual is still on the loose. This person is dangerous,” the Democrat said at a news conference.

“This is an active shooter situation right now in the city of New York.”

Fire and police officials were investigating reports that there had been an explosion, but Ms Sewell said at a press conference just after noon that there were no known explosive devices.

Multiple smoke devices were found on the scene, said mayoral spokesperson Fabien Levy.

After people streamed out of the train, quick-thinking transit workers ushered passengers to another train across the platform for safety, system chief Janno Lieber said.

No transit workers were physically hurt, according to their union.

Besides gunshot wounds, the injured riders were treated for smoke inhalation and shrapnel injuries.

President Joe Biden, vice president Kamala Harris and US attorney general Merrick Garland were briefed on the incident.

New York mayor Eric Adams, who is isolating following a positive Covid-19 test on Sunday, said in a video statement that the city “will not allow New Yorkers to be terrorised, even by a single individual”.

The incident happened on a subway line that runs through south Brooklyn in a neighbourhood predominantly home to Hispanic and Asian communities and about a 15-minute train ride to Manhattan.

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

A sea of emergency lights was visible from at least a dozen streets away, where a police cordon was set up.

New York City has faced a spate of shootings and high-profile incidents in recent months, including on the city’s subways.

One of the most shocking was in January when a woman was pushed to her death in front of a train by a stranger.

Mr Adams, a Democrat a little over 100 days into his term, has made cracking down on crime – especially on the subways – a focus of his early administration, pledging to send more police officers into stations and platforms for regular patrols.

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

Ms Hochul said: “We say: No more. No more mass shootings. No more disrupting lives. No more creating heartbreak for people just trying to live their lives as normal New Yorkers.

“It has to end.”