The hatchet-wielding man who wounded two New York City police officers, critically injuring one, was a "self-radicalised convert to Islam", US police said, believed to have acted alone in what they called a terrorist attack.

The suspect, Zale Thompson, who was shot dead by two other officers on Thursday afternoon on street in the Queens area of the city, had made anti-Western postings on social media, police told a news conference on Friday.

A search of computers seized from the home of Thompson's father, where the 32-year-old man lived, showed he also spent time reading online about beheadings, a recent intrusion at the White House and attacks in Canada.

"This was a terrorist attack," Commissioner William Bratton said.

Investigators were trying to determine whether Thompson, who converted to Islam two years ago, had any connection to a nearby mosque.

Police said Thompson's social media postings were "anti-government, anti Western, anti-white."

Still under investigation was the extent of planning involved in the attack, which appeared to be unprovoked and somewhat spontaneous, Bratton said.

"The fact that he was walking around with a hatchet in the backpack makes it clear this individual had some sense of preparation," Bratton said.

Seven-second attack

The attack on Thursday took place in a Queens shopping district in just seven seconds, police said.

A group of four police officers were posing for a freelance photographer when Thompson charged them, swinging the hatchet.

One officer was struck in the arm and another in the head before the other two officers opened fire, killing Thompson.

A female bystander was struck by a stray bullet and critically wounded.

Source: Reuters