President Donald Trump has banned the entry of Syrian refugees into the US until further notice, as part of new measures to "keep radical Islamic terrorists out of the US".
He signed a wide-ranging executive order, which also saw all refugee admissions suspended for four months.
Entry of visitors from six mainly Muslim countries to the US has also been halted for three months.
Human rights groups and high profile figures have condemned the move.
Mr Trump signed the executive order at the Pentagon after a ceremony to swear in Gen James Mattis as defence secretary.
During the ceremony, he said: "I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America. We only want to admit those into our country who will support our country and love deeply our people."
The text of the order was released several hours after it was signed. Among the measures are:
- Suspension of the US Refugee Admissions Programme for 120 days
- A ban on refugees from Syria until "significant changes" are made
- A 90-day suspension on arrivals from Iraq, Syria, and "areas of concern" - reported to include Iran, Libya, Somalia, Sudan, and Yemen
- To prioritise future refugee applications on the basis of religious-based persecution - but only if the person is part of a minority religion in their home country
- A cap of 50,000 refugees in 2017 - less than half of Mr Obama's upper limit
However, a mention of creating "safe zones" within Syria, seen in an earlier draft, was removed from the final order.
The order also said all immigration programmes should include questions to "evaluate the applicant's likelihood of becoming a positively contributing member of society."
In a TV interview broadcast earlier on Friday, the president said Christians would be given priority among Syrians who apply for refugee status in the future.
Other measures include a broad review of the information required from all countries to approve a visa; a review of visa schemes between nations to ensure they are "truly reciprocal" for US citizens; and the immediate suspension of the Visa Interview Waiver Programme.
But the document says exceptions to most restrictions could be made on a case-by-case basis.
President Trump also signed an executive order aimed at rebuilding the military by "developing a plan for new planes, new ships, new resources and new tools for our men and women in uniform".
Last year, the administration of then-President Barack Obama admitted 10,000 Syrian refugees into the US. Neighbouring Canada - whose population is a ninth of that of the US - took in 35,000.
During the presidential campaign, Mr Trump suggested a "total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on".
But he has made no mention of this since being elected last November.