A pickup driver killed eight people in New York on Tuesday, mowing down cyclists and pedestrians, before striking a school bus in what officials branded a "cowardly act of terror."

Eleven others were seriously injured in the broad daylight assault and first deadly terror-related attack in America's financial and entertainment capital since the September 11, 2001 Al-Qaeda hijackings brought down the Twin Towers.

President Donald Trump, who has curbed the number of migrants entering the United States, swiftly declared that the US "must not" allow Islamic State jihadists to "return, or enter, our country after defeating them" overseas.

The truck driver struck just blocks from the 9/11 Memorial, on the West Side of Lower Manhattan, and close to schools and a park at 3:05pm (1905 GMT) as children and their parents geared up to celebrate Halloween.

Trump decried him as "very sick" and a "deranged person." Television networks identified the 29-year-old suspect as an Uzbek citizen living in Florida. He was shot by an officer in the abdomen and taken into custody.

Police said he drove a rented Home Depot pickup down a bike and pedestrian lane, plowing into people on foot and bicycles before colliding with a school bus, injuring two adults and two children.

The suspect stepped out of the vehicle, brandishing two apparent handguns, before being shot in the abdomen by a police officer, police said.

A paintball gun and pellet gun were recovered at the scene, police said.

"This was an act of terror and a particularly cowardly act of terror aimed at innocent civilians, aimed at people going about their lives who had no idea what was about to hit them," said Mayor Bill de Blasio.

Police said eight people were killed, six of them men who died on the spot, and two others pronounced dead in hospital. Eleven other people were taken to hospital with serious, but not life-threatening injuries, officials said.

Belgium said one of its nationals was among the victims.

'Horrible tragedy'

It was the city's first terror incident since a pipe bomb exploded in September 2016 in Chelsea, lightly wounding 31 people. An American of Afghan descent, Ahmad Khan Rahimi, was convicted of terrorism on October 16 in relation with the attack.

Heavily armed police reinforcements were stepped up across the city of 8.5 million in the wake of that attack. Home to Wall Street, Broadway and one of the biggest tourist draws in the United States, it frequently goes on high alert.

A planned Halloween parade will go ahead as planned, proving that the city would not bow to threats, officials said.

Prime Minister Theresa May said she was appalled by the attack and that Britain stood with New York. "Together we will defeat the evil of terrorism," she said.

French President Emmanuel Macron also expressed solidarity. "Our fight for freedom unites us more than ever," he tweeted.

While details were preliminary and the investigation still underway, the mayor said the suspect appeared to have been a lone wolf and not part of a wider plot.

US media said the suspect shouted "Allahu akbar" and police chief James O'Neill confirmed that he made a statement when he exited the vehicle.

"If you just look at the M.O. of the attack, that's consistent with what's been going on. So that along with the statement has enabled us to label this a terrorist event," he said.

Television networks named him as Sayfullo Saipov, of Tampa, Florida. According to registry site WhitePages, a 29-year-old Sayfullo Saipov has been living in Tampa since June 2011, and had other addresses registered in Ohio.

He was listed as having had several traffic-related minor violations.

In New York, police officers and ambulances swamped the area, in front of a park and next to a school as sirens wailed continuously and helicopters roared through the sky.

Worried parents gathered outside a public elementary school that had closed, waiting to see if they could collect children who stayed after the end of the classes for extracurricular lessons and activities.

'Feel the pain'

John Williams, 22, on his way to a park at the time, said he did not witness the shooting, but arrived 30 seconds afterward.

"There was a smell of gunshots," he told AFP. "There was a man lying on the ground. It looked as if he'd been shot."

State Governor Andrew Cuomo said the city was a "target" as an "international symbol of freedom and democracy."

"We've lived with this before, we've felt the pain before. We feel the pain today. But we go forward together," he added.

A witness who gave his name only as Frank told local television network NY1 that he saw a man running around an intersection, heard five to six gunshots and saw "about 100 cops" flood into the street.

"When the cops shot him, everybody started running away and it got a little bit crazy right there. So when I tried to look again, the guy was already down," the witness said.

Tuesday's attack came five months after a US Navy veteran plowed a car into pedestrians in Times Square, killing an 18-year-old woman from Michigan and injuring 22 other people on May 18 in what de Blasio said was not an act of terror.

Previously, the most serious security breach in New York since Faisal Shahzad, a Pakistani immigrant, plant a car bomb in Times Square on May 1, 2010.

His explosive device failed to detonate and he was arrested shortly after boarding a flight to the Middle East. He pleaded guilty and said he was aiming to avenge deaths from US missiles fired from drones operating over Pakistan.

He was sentenced to life behind bars.