Donald Trump has barred all refugees from entering the United States for four months - and those from war-ravaged Syria indefinitely - to stop "radical Islamic terrorists".

The president's order immediately suspended a programme that resettled to the US about 85,000 people displaced by war, political oppression, hunger and religious prejudice last year.

And Mr Trump indefinitely blocked all those fleeing Syria, where a civil war has displaced millions of people, and imposed a 90-day ban on entry to the US from seven Muslim-majority nations.

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"We want to ensure that we are not admitting into our country the very threats our soldiers are fighting overseas," Mr Trump said as he signed the order at the Pentagon.

"We only want to admit those into our country who will support our country and love deeply our people."

He said the halt to the refugee programme was necessary to give government agencies time to develop a stricter vetting system - but the order spells out what additional steps he wants the homeland security and state departments to take.

The US may admit refugees on a case-by-case basis during the freeze and the government will continue to process requests from people claiming religious persecution "provided that the religion of the individual is a minority religion in the individual's country".

In an interview with CBN News, Mr Trump said persecuted Christians would be given priority in applying for refugee status.

"We are going to help them," he said. "They've been horribly treated."

The order was signed on Mr Trump's most robust day of national security and foreign policy at the start of his presidency, marked by a meeting with British prime minister Theresa May and a lengthy phone call with Mexican president Enrique Pena Nieto.

As a presidential candidate, Mr Trump called for a temporary ban on all Muslim immigration to the US and later shifted his focus to putting in place "extreme vetting" procedures to screen people coming from countries with terrorism ties.

The State Department said the three-month ban in the directive applied to Iraq, Syria, Iran, Sudan, Libya, Somalia and Yemen - all Muslim-majority nations.

The Council on American-Islamic Relations (CAIR), said it would file a legal action on Monday, challenging the constitutionality of the executive order.

"There is no evidence that refugees - the most thoroughly vetted of all people entering our nation - are a threat to national security," said the group's national litigation director Lena Masri.

"This is an order that is based on bigotry, not reality."

During the past budget year, the US accepted 84,995 refugees, including 12,587 people from Syria.

Barack Obama had set the refugee limit for this budget year at 110,000 but Mr Trump, according to the executive order, plans to slash that to 50,000.

Refugee processing was suspended in the immediate aftermath of the September 11 2001 attacks and restarted months later.

The president was applauded by House of Representatives speaker Paul Ryan, who said it was "time to re-evaluate and strengthen the visa-vetting process".

But many Democrats cast the measures as un-American.

"Tears are running down the cheeks of the Statue of Liberty tonight as a grand tradition of America, welcoming immigrants, that has existed since America was founded has been stomped upon," said Senate minority leader Chuck Schumer.

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Mr Trump's order was signed on Holocaust Remembrance Day, which brought to mind the global effort to help refugees during the Second World War and its aftermath.

The order makes no mention of a plan to provide safe zones in Syria and the surrounding area.

The president's directive capped a hectic first week for Mr Trump at the White House, giving Americans an initial look at how he intends to position the United States around the world.

Earlier, he hosted Mrs May at the White House for his first meeting with a world leader since taking office.

Asked about whether he would revert to Bush-era use of torture, Mr Trump said he would defer to the views of defence secretary Jim Mattis.

"He has stated publicly that he does not necessarily believe in torture or waterboarding, or however you want to define it. I don't necessarily agree," Mr Trump said.

"But I would tell you that he will override because I'm giving him that power. He's an expert."

News organisations have obtained copies of a draft executive order signalling sweeping changes to US interrogation and detention policy.

The draft, which the White House said was not official, also requests recommendations on whether the US should reopen CIA detention facilities outside the United States.

Critics said the clandestine sites have marred America's image on the world stage.

Mr Trump also held firm on Friday on another controversy - trade and illegal immigration from Mexico.

He told reporters he had a "very good call" with Mr Pena Nieto earlier in the day, but he reaffirmed his belief that Mexico had "out-negotiated and beat us to a pulp" on trade, and that would change.

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"We're no longer going to be the country that doesn't know what it's doing," he declared, a day after the Mexican leader cancelled his visit to Washington in response to Mr Trump's plans to build a border wall and make Mexico pay for it.