A damaged school bus is seen at the scene of a pickup truck attack in Manhattan, New York, US, Oct 31, 2017 in this picture obtained from social media. [Photo/Agencies]

New York and other cities around the globe have been on high alert against attacks by extremists in vehicles. The Islamic State has been exhorting its followers to mow down people, and England, France and Germany have seen deadly vehicle attacks in the past year or so.

New York Gov. Andrew Cuomo called it a "lone wolf" attack and said there was no evidence to suggest it was part of a wider plot.

City Police Commissioner James O'Neill said a statement the driver made as he got out of the truck and the method of attack led police to conclude it was a terrorist act.

On Twitter, US President Donald Trump called it "another attack by a very sick and deranged person" and declared, "NOT IN THE USA.!" While police did not specifically blame the Islamic State for the New York bloodshed, Trump railed against the extremist group, tweeting, "We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"

Police said Saipov rented the truck at about 2 pm in New Jersey, entering the bike path about an hour later on West Street a few blocks from the new World Trade Center, the site of the deadliest terror attack in US history. The truck then turned at Chambers Street, hitting the school bus and injuring two adults and two children.

A paintball gun and a pellet gun were found at the scene, police said. At least two covered-over bodies could be seen lying on the bike path, and the front end of the truck was smashed in, as was the side of the school bus.

Tom Gay, a school photographer, heard people saying there was an accident and went down to West Street, where a woman came around the corner shouting, "He has a gun! He has a gun!" Gay said he stuck his head around the corner and saw a slender man in a blue tracksuit running on West Street holding a gun. He said a heavyset man was chasing him.

He said he heard five or six shots, and the man in the tracksuit fell to the ground, gun still raised in the air. He said a man came over and kicked the gun out of his hand.

AP