U.S. President Donald Trump on Monday signed a new version of his controversial travel ban, aiming to withstand court challenges while still barring new visas for citizens from six Muslim-majority countries and putting restrictions on the U.S. refugee program.

The revised travel order leaves Iraq off the list of banned countries but still affects would-be visitors from Iran, Syria, Somalia, Sudan, Yemen and Libya.

.<a href="https://twitter.com/POTUS">@POTUS</a> has signed a new executive order that will keep the nation safe. Read it at <a href="https://t.co/XcBwwEJJHF">https://t.co/XcBwwEJJHF</a> <a href="https://t.co/Yx2mikXl2A">pic.twitter.com/Yx2mikXl2A</a> —@PressSec

Trump privately signed the new order while Homeland Security Secretary John Kelly, Secretary of State Rex Tillerson and Attorney General Jeff Sessions formally unveiled the new edict.

The low-key roll-out was a contrast to the first version of the order, signed in a high-profile ceremony at the Pentagon's Hall of Heroes as Secretary of Defence James Mattis stood by Trump's side.

White House Press Secretary Sean Spicer was not scheduled to hold an on-camera briefing Monday either, leading to the appearance that the president was distancing himself from the order, which was a signature issue during his campaign and the first days of his presidency. The order also risks being overshadowed by unsubstantiated accusations the president made over the weekend that former President Barack Obama had ordered the wiretapping of his phone during the campaign.

Beth Kohn protested outside the 9th U.S. Circuit Court of Appeals courthouse in San Francisco on Feb. 7 while the court heard arguments regarding President Donald Trump's temporary travel ban on people from seven Muslim-majority countries. (Noah Berger/Reuters)

The original travel ban caused immediate panic and chaos at airports around the country as Homeland Security officials scrambled to interpret how it was to be implemented and travellers were detained before being sent back overseas or blocked from getting on airplanes abroad. The order quickly became the subject of several legal challenges and was ultimately put on hold last month by a federal judge in Washington state. That ruling was upheld by a federal Appeals Court.

The revised order is narrower and specifies that a 90-day ban on people from the six countries does not apply to those who already have valid visas or people with U.S. green cards.

The White House dropped Iraq from the list of targeted countries following pressure from the Pentagon and State Department, which had urged the White House to reconsider, given Iraq's key role in fighting the Islamic State group. Syrian nationals are also no longer subjected to an indefinite ban, despite Trump's insistence as a candidate that Syrian refugees in particular posed a serious security threat to the United States.

The new executive order says the U.S. refugee program will be capped at 50,000 for 2017. (Ted Soqui/Reuters)

In a call with reporters Monday morning, senior officials from Homeland Security and Justice Department said the travel ban was necessary to allow the government to review what more can be done to properly vet would-be visitors and refugees.

The officials said 300 people who arrived in the United States as refugees were currently under investigation as part of terrorism-related cases. The officials pointed to those cases as evidence of the need for the travel order, but refused repeated requests to address how many of those people were from the six banned countries or how long they have been in the United States.

David Miliband, president of the International Rescue Committee, said the cut in refugee admissions targets the vulnerable.

"[The order is a] historic assault on refugee resettlement to the United States, and a really catastrophic cut at a time there are more refugees around the world than ever before," Miliband told the Associated Press.

Immigration lawyer Warren Creates on how Canada could benefit from tighter U.S. borders 6:25

A fact sheet describing the order circulated before the new order was announced cites negotiations that resulted in Iraq agreeing to "increase co-operation with the U.S. government on the vetting of its citizens applying for a visa to travel to the United States."

The mere existence of a fact sheet signalled that the White House was taking steps to improve the roll-out of the reworked directive. The initial measure was hastily signed at the end of Trump's first week in office, and the White House was roundly criticized for not providing lawmakers, Cabinet officials and others with information ahead of the signing.

Trump administration officials say that even with the changes, the goal of the new order is the same as the first: keeping would-be terrorists out of the United States while the government reviews the vetting system for refugees and visa applicants from certain parts of the world.

According to the fact sheet, the Department of Homeland Security will conduct a country-by-country review of the information the six targeted nations provide to the U.S. for visa and immigration decisions. Those countries will then have 50 days to comply with U.S. government requests to update or improve that information.

Additionally, Trump's order suspends the entire U.S. refugee program for 120 days, though refugees already formally scheduled for travel by the State Department will be allowed entry. When the suspension is lifted, the number of refugees allowed into the U.S. will be capped at 50,000 for fiscal year 2017.

Language removed on religious minorities

The new version also removes language that would give priority to religious minorities. Critics had accused the administration of adding such language to help Christians get into the U.S. while excluding Muslims.

"I think people will see six or seven major points about this executive order that do clarify who was covered," said presidential counsellor Kellyanne Conway in an interview with Fox News Channel's Fox & Friends.

She said the new order will not go into effect until March 16, despite earlier warnings from the president and his team that any delay in implementation would pose a national security risk, allowing dangerous people to flow into the country.

Attorney General Jeff Sessions says the president has the legal authority to protect the U.S. with his executive order 0:50

Republican Senator Lindsey Graham offered his support for the revised ban.

"I believe the new order will withstand legal challenges as it's drafted in a fashion as to not be a religious ban, but a ban on individuals coming from compromised governments and failed states. This executive order will help achieve President Trump's goal of making us safer," he said.

Legal experts say the new order addresses some of the constitutional concerns raised by a federal appeals court about the initial ban, but leaves room for more legal challenges.

"It's much clearer about how it doesn't apply to groups of immigrants with more clearly established constitutional rights," said University of Texas law professor Stephen Vladeck. "That's a really important step."

Economist Dambisa Moyo on what the new U.S. policy means for the global economy 8:10

Removing language that would give priority to religious minorities helps address concerns that the initial ban was discriminatory, but its continued focus on Muslim-majority countries leaves the appearance that the order is a "Muslim ban," Vladeck said.

The American Civil Liberties Union says it will respond to the new order, either by amending existing lawsuits or by seeking a new injuction.

"We're going to move very quickly in court to make sure that at least one of the injunctions currently in place around the country remains in place," said Lee Gelernt, deputy director of the ACLU's Immigrants Rights Project.