Trump travel ban blocking citizens from 12 countries takes effect

The ban, signed last week, applies to citizens of Afghanistan, Burma, Chad, the Republic of Congo, Equatorial Guinea, Eritrea, Haiti, Iran, Libya, Somalia, Sudan and Yemen.

It also imposes heightened restrictions on people from Burundi, Cuba, Laos, Sierra Leone, Togo, Turkmenistan and Venezuela who are outside the U.S. and don't hold a valid visa.

The order does not cancel visas that have already been issued to citizens of the countries, and those who possess them will remain free to travel to the U.S.

This point was backed by the State Department's principal deputy spokesperson, Tommy Pigott. In a briefing last week, Pigott said, "This is a national security imperative."

"We cannot have open migration from any country where we cannot safely and reliably vet and screen those who seek to enter the United States," Trump said before signing the ban.

"That is why today I am signing a new executive order placing travel restrictions on countries including Yemen, Somalia, Haiti, Libya and numerous others," he concluded. "We will not allow people to enter our country who wish to do us harm, and nothing will stop us from keeping America safe."

Fox News' Paul Tilsley and the Associated Press contributed to this report.