President Donald Trump has banned the entry of Syrian refugees into the US until further notice.
He has also halted the issuing of visas to the nationals of six other mainly Muslim countries, including Iran, Iraq, Yemen and Libya, for three months.
Mr Trump said the measures were part of new measures to "keep radical Islamic terrorists out of the US".
Rights groups have condemned the move, saying there is no link between Syrian refugees in the US and terrorism.
Under Mr Trump's wide-ranging executive order, all refugee admissions have been suspended for four months.
Mr Trump signed the 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 anyone arriving from Iraq, Syria, Iran, Libya, Somalia, Sudan, and Yemen, except certain visa categories such as diplomats
To prioritise future refugee applications from those persecuted for their religion - 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 the upper limit under Mr Trump's predecessor, Barack Obama
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.