Donald Trump has signed an executive action for new vetting measures he says is aimed at keeping "radical Islamic terrorists" out of the US.
He has halted refugee arrivals from Syria to the US and imposed a 90-day ban on entry to the US from seven Muslim majority nations including Iraq, Syria, Iran, Sudan, Libya, Somalia and Yemen.
The president signed the order at the Pentagon, saying "we don't want them here."
"I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America," Trump said.
"We only want to admit those into our country who will support our country and love deeply our people."
What has Mr Trump ordered?
The State Department will stop issuing visas to Syrian nationals and halts the processing of Syrian refugees.
That will remain in effect until Trump determines that enough security changes have been made to ensure that would-be terrorists can't exploit weaknesses in the current vetting system.
Trump ordered a four-month suspension to the US's broader refugee programme.
The suspension is intended to provide time to review how refugees are vetted before they are allowed to resettle in the United States.
Trump's order also cuts the number of refugees the United States plans to accept this budget year by more than half, to 50,000 people from around the world.
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."
Trump's order will direct officials to review the refugee application and approval process to find any other security measures that can be added to prevent people who pose a threat from using the refugee program.
During the Obama administration, vetting for refugees included in-person interviews overseas, where they provided biographical details about themselves, including their families, friendships, social or political activities, employment, phone numbers, email accounts and more.
They also provided biometric information, including fingerprints. Syrians were subject to additional, classified controls that administration officials at the time declined to describe, and processing for that group routinely took years to complete.
He has also imposed a 90-day ban on entry to the US from seven Muslim majority nations including Iraq, Syria, Iran, Sudan, Libya, Somalia and Yemen.
The order also calls for a review of what information the government needs to fully vet would-be visitors and come up with a list of countries that don't provide it.
The order says the government will give countries 60 days to start providing the information or citizens from those countries will be barred from travelling to the United States.
What has the reaction been?
Google has reportedly recalled travelling staff members to the US after Mr Trump's executive order restricting entry for nationals of seven Muslim-majority countries.
A Google spokesperson told ITV News they are "concerned about the impact of this order" and any proposals that could impose restrictions on their employees and their families.
The United Nations refugee agency and International Organioation for Migration (IOM) called on Trump to continue offering asylum topeople fleeing war and persecution, saying its resettlement programme was vital.
"The needs of refugees and migrants worldwide have never been greater and the US resettlement program is one of the most important in the world," the two Geneva-based agencies said in a joint statement.
The French foreign minister, Jean-Marc Ayrault, said: "This can only worry us. Welcoming refugees who flee war and oppression is part of our duty."
Malala Yousafzai, the teenage Nobel Prize winner and activist said she is "heartbroken" over Trump's plans to restrict immigrants and refugees entering the US.
The International Rescue Committee is calling President Donald Trump's suspension of the US refugee resettlement program a "harmful and hasty" decision.
IRC President David Miliband said: "America must remain true to its core values. America must remain a beacon of hope."
The IRC statement declared that the US vetting process for prospective refugees is already robust involving biometric screening and up to 36 months of vetting by "12 to 15 government agencies".
Facebook founder Mark Zuckerberg said the US is a "nation of immigrants" and "we should be proud of that".