Washington (AFP) - US President Donald Trump unleashed a wave of alarm Saturday with his order to temporarily halt all refugee arrivals and impose tough controls on travelers from seven Muslim countries including war-wracked Syria.

Making good on one of his most controversial campaign promises, and to the horror of human rights groups, Trump said he was making America safe from "radical Islamic terrorists."

"This is big stuff," he declared at the Pentagon, after signing an executive order entitled "Protection of the nation from foreign terrorist entry into the United States."

Trump's decree suspends the entire US refugee resettlement program for at least 120 days while tough vetting rules are established.

These new protocols will "ensure that those approved for refugee admission do not pose a threat to the security and welfare of the United States."

In addition, it specifically bars Syrian refugees from the United States indefinitely, or until the president himself decides that they no longer pose a threat.

Meanwhile, no visas will be issued for 90 days to migrants or visitors from seven mainly-Muslim countries: Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.

In reaction to the order the United Nations on Saturday urged the US to continue its "strong leadership role and long tradition of protecting those fleeing conflict and persecution."

In a joint statement the UN refugee agency, UNHCR, and the International Organization for Migration said they believed "refugees should receive equal treatment for protection and assistance, and opportunities for resettlement, regardless of their religion, nationality or race."

- Extreme vetting -

The order laid the way for what Trump has pledged will be the "extreme vetting" of visa applicants' backgrounds -- with some exceptions made for members of "religious minorities," a caveat many see as a way to apply favorable treatment to Christians from majority-Muslim states.

Civil liberties groups and many counterterrorism experts condemned the measures, declaring it inhumane to lump the victims of conflict in with the extremists who threaten them.

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"'Extreme vetting' is just a euphemism for discriminating against Muslims," said Anthony Romero, executive director of the American Civil Liberties Union.

Romero said Trump's order breached the US constitution's ban on religious discrimination by choosing countries with Muslim majorities for tougher treatment.

Ahmed Rehab, director of the Chicago chapter of the Council on American-Islamic Relations, told AFP his group would mount legal challenges to fight the order "tooth and nail."

"It is targeting people based on their faith and national origin, and not on their character or their criminality," he told AFP.

Speaking in Tehran Saturday, Iranian President Hassan Rouhani did not comment directly on the visa ban but said Iran had "opened its doors" to foreign tourists since the signing of a nuclear agreement with world powers in 2015.

He also said now was "not the time to build walls between nations," a reference to Trump's plans to build a wall along the US-Mexico border.

But the US leader did get backing from Czech President Milos Zeman, who praised him for being "concerned with the safety of his citizens".

Zeman, a vocal supporter of Trump, has denounced Syrian refugee arrivals in his own country as "an organized invasion" and said Muslims were "impossible to integrate".

- The most vulnerable -

The travel controls will be popular with Trump's nationalist base, though they stop short of a threat made during last year's campaign to halt all Muslim travel to the United States.

Trump's supporters defend the measures as necessary to prevent supporters of Al-Qaeda or the Islamic State group from infiltrating the US homeland disguised as refugees.

The US State Department, which with the Department of Homeland Security will have to implement the policies, said it was ready to put them into immediate effect.

"We will announce any changes affecting travelers to the United States as soon as that information is available," spokesman Mark Toner said.

"We take seriously our responsibility to safeguard the American public while remaining committed to assisting the world's most vulnerable people."

Trump's order also cut the number of refugees the United States plans to resettle in fiscal year 2017 -- which is calculated from last October on -- from 110,000 to 50,000.

During the ceremony, he also signed an order to "rebuild" the US military and watched Vice President Mike Pence swear in former Marine general James Mattis as his new secretary of defense.

Earlier Trump had admitted he would allow the general's opposition to the use of torture to override his own enthusiasm for harsh measures.

- 'Wonderful thing' -

Trump also met Friday with Britain's Prime Minister Theresa May, the first foreign leader to visit his White House since his inauguration.

He praised Britain's decision to leave the European Union as a "wonderful thing."

Over the weekend, Trump is due to hold calls with Japanese Prime Minister Shinzo Abe, Germany's Chancellor Angela Merkel, Russia's President Vladimir Putin, France's President Francois Hollande and Australian Prime Minister Malcolm Turnbull.

He is keen to develop friendly ties with Moscow, but has played down reports that he might quickly end US economic sanctions imposed on Russia for its intervention in Ukraine.