U.S. President Donald Trump said on Twitter he had ordered the Department of Homeland Security to step up its "extreme vetting program" following a vehicle attack in New York.

"I have just ordered Homeland Security to step up our already Extreme Vetting Program. Being politically correct is fine, but not for this!" Trump said, hours after a man driving a pickup truck killed eight people.

READ: Eight dead in truck attack on Manhattan bike path; suspect arrested

Trump ran for office in 2016 pledging to crack down on illegal immigration for security reasons, and has called for "extreme vetting" of foreigners entering the United States.