President Donald Trump slapped new travel restrictions on citizens from North Korea, Venezuela and Chad, expanding the list of countries covered by his original travel bans that have been derided by critics as targeting Muslims.

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

"We cannot afford to continue the failed policies of the past, which present an unacceptable danger to our country," Trump said in statement. "My highest obligation is to ensure the safety and security of the American people, and in issuing this new travel order, I am fulfilling that sacred obligation."

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.

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The new restrictions, slated to take effect on October 18, resulted from a review after Trump's original travel bans sparked international outrage and legal challenges.

Unlike the first ban - which sparked chaos at airports across the country - officials said they had been working for months on the new rules, in collaboration with various agencies and in conversation with foreign governments.

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

Critics have accused the president of discriminating against Muslims in violation of constitutional guarantees of religious liberty and equal protection under the law, breaking existing US immigration law and stoking religious hatred.

Trump had called for a "total and complete shutdown of Muslims entering the United States" during his election campaign.

Speaking on background, government officials said the addition of North Korea and Venezuela demonstrated the measure was set on the basis of security and was not a "Muslim ban", as detractors have argued.

"Religion, or the religious origin of individuals or nations, was not a factor," a senior government official told reporters.

"The inclusion of those countries, Venezuela and North Korea, was about the fact that those governments are simply not compliant with our basic security requirements."

Rights group Amnesty International USA condemned the new 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".

The National Iranian American Council (NIAC) denounced the new proclamation as "nothing but an extension of the same discrimatory policy first rolled out in January".

"The Trump administration has now taken steps to make its Muslim ban targeting Iranians and other nationals permanent," NIAC said in statement.

"Absent additional intervention from the courts, and a long-overdue intervention from the Republican-controlled Congress, the Trump administration will cement a racist and discriminatory campaign promise into official US policy."

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The Supreme Court will hear arguments on October 10 on whether the current ban discriminates against Muslims in violation of the US Constitution, as lower courts previously ruled.

Now the nine-justice court could skip deciding the case altogether, legal experts said.

With the travel restrictions expiring, the court has an easy way out because it could simply say the case is no longer a live issue and therefore, in legal parlance, moot.

"If the court can avoid entering into the fray, that may be appealing to them," said Anil Kalhan, an immigration law professor at Drexel University School of Law.