WASHINGTON, Oct. 31 (Xinhua) -- U.S. President Donald Trump said Tuesday that he has ordered to step up an "extreme vetting program", after a terrorist attack in New York city killed eight people.

"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 in a tweet, without specifying which vetting program he was referring to

He has repeatedly called for tighter border control and visa policies to keep out those deemed dangerous to U.S. national security.

White House Chief of Staff John Kelly described extreme vetting during a media interview on Monday, saying "extreme vetting is, we simply interview people and have to satisfy ourselves that the person we are talking to is indeed the person who they claim."

"If we can't verify, I don't think we should let them into the country," Kelly said.

The Trump administration has rolled out multiple travel bans against Muslim-majority countries and in one case against the Democratic People's Republic of Korea and Venezuela, but faced strong backlash within the country, as activists claim such bans are discriminative measures targeting a specific religion.