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President Donald Trump on Friday signed an executive order temporarily halting a U.S. refugee program and indefinitely suspending admission of Syrian refugees, and issuing 90-day restrictions that could affect people from seven predominantly-Muslim nations.

The order suspends the admittance of Syrian refugees indefinitely until changes are made to U.S. policies. The move follows Trump's campaign pledges to institute "extreme vetting."

In signing the order, Trump vowed to "keep radical Islamic terrorists out of the United States of America." Trump also signed a second order aimed at "rebuilding the U.S. Armed Forces."

"We are not admitting into the country the very threats our soldiers are fighting overseas," Trump said during the swearing-in ceremony for Defense Secretary James Mattis at the Pentagon. "We only want to admit those into our country who will support our country and love deeply our people."

Islam is not mentioned in the order, but critics condemned it as a "Muslim ban" and at least one group promised a challenge on constitutional grounds.

The order specifically singles out the arrival of Syrian refugees as "detrimental to the interests of the United States" and suspends their entry to the country until Trump is satisfied with changes to refugee programs.

This is a complete reversal of a hotly debated policy set forth by the Obama administration that made it a goal last year of admitting 10,000 Syrian refugees driven by the violence that occurred during their nation's protracted civil war. America reached that goal in August.

While Syria is the only country singled out by name, the order suspends entry for 90 days from certain nations based on statute related to the Visa Waiver Program.

The most recent version of that visa waiver program put restrictions on those who had traveled to seven countries since March of 2011: Iraq, Syria, Iran, Sudan, Libya, Somalia, or Yemen. Some visa exemptions are included in Trump's order.

The order also leaves open the possibility of including other nations. The order suspends all refugee admissions for 120 days while the visa program is under review.

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An exception is made for allowing officials to "prioritize refugee claims on the basis of religious-based persecution" for religious minorities in those country.

The order limits the total number of refugees to 50,000 in fiscal year 2017 and suspends waivers of in-person interviews for visas.

The State Department in a statement on Friday said it is working with the Departments of Homeland Security and Health and Human Services to put Trump's executive order into effect.

And while no particular religion is mentioned, the order specifically identifies "those who would place violent ideologies over American law."

"In addition, the United States should not admit those who engage in acts of bigotry or hatred (including 'honor' killings, other forms of violence against women, or the persecution of those who practice religions different from their own) or those who would oppress Americans of any race, gender, or sexual orientation," the order says.

Extreme vetting is an idea that evolved from Trump's controversial Muslim ban that he called for in December 2015. It was intended to address concerns from some over refugees entering the country from terror-stricken nations.

"We've taken in tens of thousands of people. We know nothing about them,” Trump said in an interview with Fox News’ Sean Hannity on Thursday. “They can say they vet them. They didn’t vet them. They have no papers. How can you vet somebody when you don’t know anything about them and you have no papers?"

Though the United States government already utilizes a thorough nine-step vetting system for refugees, the president's executive order is said to go further. It is still unclear what these steps would mean and if it would target any particular religious group as the president stated on the campaign trail.

According to the Pew Research Center, the United States welcomed almost 85,000 refugees in 2016. Forty-six percent of them were Muslim.

The president sets the number of refugees accepted annually into the U.S. and, from that authority, can suspend the program at any time. President George W. Bush took similar actions following the 9/11 attacks when the program was suspended for several months.

Rep. Mike McCaul, R-Texas, chairman of the Homeland Security committee, said on Friday that he applauds the president's move.

"We are a compassionate nation and a country of immigrants. But as we know, terrorists are dead set on using our immigration and refugee programs as a Trojan Horse to attack us," McCaul said in a statement. "Today, President Trump signed an order to help prevent jihadists from infiltrating the United States."

The response from advocates for refugee rights was swift.

"I am heartbroken that today President Trump is closing the door on children, mothers and fathers fleeing violence and war," Malala Yousafzai, a student activist and the youngest-ever Nobel Prize laureate said in a statement through a press office. "I am heartbroken that America is turning its back on a proud history of welcoming refugees and immigrants — the people who helped build your country, ready to work hard in exchange for a fair chance at a new life."

Facebook CEO Mark Zuckerberg took to the social media platform to express his concern about the executive order's impact on those fleeing violence and oppression in their homelands. His great-grandparents came from Germany, Austria and Poland. The parents of his wife, Priscilla, were refugees from China and Vietnam.

"We need to keep this country safe, but we should do that by focusing on people who actually pose a threat. Expanding the focus of law enforcement beyond people who are real threats would make all Americans less safe by diverting resources, while millions of undocumented folks who don't pose a threat will live in fear of deportation," he wrote. "We should also keep our doors open to refugees and those who need help. That's who we are. Had we turned away refugees a few decades ago, Priscilla's family wouldn't be here today."

Trump also called for a "great rebuilding" of the nation's military. "New planes, new ships, new resources, new tools for our men and women in uniform," the president said.

The military executive order instructs the defense secretary to conduct a 30-day “readiness review,” and along with the Office of Management and Budget, develop a budget amendment.

The order also instructs Mattis to conduct “a new nuclear posture review” and “a new ballistic missile defense review.”

The nuclear posture review is aimed to “ensure the United States nuclear deterrent is modern, robust, flexible, resilient, ready, and appropriately tailored to deter 21st-century threats and reassure our allies,” according to the order.