The president of the United States Tuesday night ordered extreme vetting of foreign nationals after it was revealed that the perpetrator of an attack in New York City that killed eight people was an immigrant from Uzbekistan.

"I have just ordered Homeland Security to step up our already Extreme Vetting Program. Being politically correct is fine, but not for this!" Donald Trump wrote on Twitter.

Trump did not specify which program he was referring to in his tweet nor the measures it entailed.

The White House also did not offer any explanations regarding the announcement.

A few hours earlier, the president had referred to the Islamic State terror group in another tweet although the organization has not yet claimed the New York City attack.

"We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!" Trump wrote.

The US media identified the suspect of Tuesday's multiple hit-and-run in New York City as Sayfullo Saipov, a 29-year-old Uzbek national.

According to ABC, Saipov entered the US seven years ago under a program which offers a lottery for people from countries with few immigrants in America.

According to witnesses of the incident, the attacker yelled "Allahu Akbar" ("God is great") when he came out of the pickup truck he was driving after it crashed into a school bus after he had run over dozens of people.