The pickup truck driver who plowed down a New York cycle path, killing eight people, in the city's worst attack since September 11, was associated with the militant Islamic State (IS) group but “radicalised domestically,” the state's governor said on Wednesday.

The driver, identified as Uzbek national named Sayfullo Saipov, mowed down pedestrians and cyclists over a mile-long stretch of the bike path on Tuesday in an exclusive neighbourhood of Lower's Manhattan's West Side, where children and their parents were preparing to celebrate Halloween.

Police shot Saipov, 29, in the stomach at the end of the rampage, but he was expected to survive.

Handwritten notes in Arabic were found near his vehicle, pledging allegiance to the militant group, the New York Times reported, citing law enforcement officials. Documents found inside the truck included a picture of an IS flag, the New York Post said.

“He's a depraved coward is what he is, and he was associated with ISIS, and he was radicalised domestically, and he's a depraved coward,” New York Governor Andrew Cuomo told CNN.

Cuomo said investigators had determined that the suspect, who reportedly entered the United States in 2010, had since shown an interest in IS and its terror tactics, but that there was no evidence so far that others helped him plot the attack.

“Our only evidence to date is that this was an isolated incident that he, himself, performed,” the governor said.

Tuesday's truck rampage struck in broad daylight just blocks from the memorial to the September 11, 2001 attacks.

Five of the eight killed were Argentine nationals, part of a group visiting New York from the city of Rosario for a school reunion, the foreign ministry in Buenos Aires said.

Brussels said a Belgian woman on a trip with her mother and sister was killed.

Eleven people, including an Argentine and three Belgians, were also wounded.

'Extreme vetting'

President Donald Trump has denounced the attacker as “very sick” and a “deranged person,” and announced a stepping-up of his “extreme vetting program” for foreign travelers to the country.

The United States “must not” allow members of the IS to “return, or enter” the country after being defeated overseas, Trump said.

On Wednesday the president tweeted that the suspect had entered the United States via a visa lottery program supported by opposition Democrats.

“We are fighting hard for Merit Based immigration, no more Democrat Lottery Systems. We must get MUCH tougher (and smarter),” Trump tweeted.

Saipov was living in Tampa, Florida and had recently been staying in New Jersey, where the truck was rented, reports said. Law enforcement sources said he was arrested in Missouri on a traffic violation last year.

Police said he drove a rented Home Depot pickup down a bike and pedestrian lane where tourists and New Yorkers were out enjoying brilliant fall sunshine, just after 3 pm, before colliding with a school bus, wounding two adults and two children.

The suspect then fled the vehicle brandishing weapons that were subsequently identified as a paintball gun and pellet gun, before being shot in the abdomen by a police officer and taken into custody.

New York police chief James O'Neill confirmed he made a statement when he exited the vehicle that had led investigators to “label this a terrorist event.”

'Little bit crazy'

The attacker struck in TriBeCa, one of the city's most upscale neighborhoods.

“There was a smell of gunshots,” said John Williams, 22, who arrived at the scene 30 seconds afterward en route to the park. “There was a man lying on the ground. It looked as if he'd been shot.”

“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,” a witness who gave his name only as Frank told local television network NY1.

Heavily-armed police fanned out across the city of 8.5 million, home to Wall Street, Broadway and one of the biggest tourist draws in the United States.

Tuesday's attack was the first deadly terror-related incident in the US financial and entertainment capital since Al-Qaeda hijackers brought down the Twin Towers, killing more than 2,700 people on 9/11.

But a planned Halloween parade went ahead as scheduled under tight security, and Governor Cuomo has vowed the city would not be cowed by the latest attack, ordering the new World Trade Center to be lit red, white and blue “in honor of freedom and democracy. “