President Donald Trump has slapped new travel restrictions on citizens from North Korea, Venezuela and Chad, expanding to eight the list of countries covered by his original travel bans that have been derided by critics and challenged in court.

Iran, Libya, Syria, Yemen and Somalia were left on the list of affected countries in a new proclamation issued by the president. Restrictions on citizens from Sudan were lifted.

The measures help fulfill a campaign promise Trump made to tighten US immigration procedures and align with his "America First" foreign policy vision. Unlike the president's original bans, which had time limits, this one is open- ended.

Making America Safe is my number one priority. We will not admit those into our country we cannot safely vet.https://t.co/KJ886okyfC

— Donald J. Trump (@realDonaldTrump) September 24, 2017

Iraqi citizens will not be subject to travel prohibitions but will face enhanced scrutiny or vetting.

The current ban, enacted in March, was set to expire on Sunday evening. The new restrictions are slated to take effect on October 18 and resulted from a review after Trump's original travel bans sparked international outrage and legal challenges.

The addition of North Korea and Venezuela broadens the restrictions from the original, mostly Muslim-majority list.

An administration official, briefing reporters on a conference call, acknowledged that the number of North Koreans now travelling to the United States was very low.

Rights group Amnesty International USA condemned the measures.

"Just because the original ban was especially outrageous does not mean we should stand for yet another version of government-sanctioned discrimination," it said in a statement.

"It is senseless and cruel to ban whole nationalities of people who are often fleeing the very same violence that the US government wishes to keep out. This must not be normalised."

The American Civil Liberties Union said in a statement the addition of North Korea and Venezuela "doesn't obfuscate the real fact that the administration's order is still a Muslim ban."