More than 18,000 have been admitted as refugees since October 2011. But the Trump administration has significantly thinned the flow of refugees, and officials said this week that those from Syria and several other countries will be let in only after undergoing new vetting on top of the screening they already receive.

The temporary status is granted to certain groups of people in the wake of natural disasters, wars, outbreaks of disease and other catastrophes that would make it difficult for them to return safely to their home countries. Government officials periodically review the program to decide whether to extend it, and most groups have received regular extensions in the past. The administration ended protected status for people from Haiti and El Salvador on the grounds that both countries had recovered sufficiently from the earthquakes that were the reason for their original inclusion in the program.

The protections can be extended for 6, 12, or 18 months at a time. The Obama administration extended them for Syrians three times, and each time it allowed new applicants to join. Homeland security officials did not say on Wednesday evening why they had decided to offer an extension to Syrians who already had the status, but not to offer it to those who did not.

Syria was originally included in the program in 2012 because of the armed conflict among government forces, anti-government insurgents and the Islamic State. According to the United Nations refugee agency, more than 5.4 million people have fled Syria since 2011, when the civil war began, and that number climbs every month. Another 6.1 million Syrians have been forced from their homes, but are still living within Syria’s borders.

Just this month, the United States accused the government of Syria’s president, Bashar al-Assad, of attacking its own people with chemical weapons, a war crime, in the same region that was devastated four years ago by the deadliest chemical attacks of the entire Syrian conflict.

“There is no way to go back,” said Radwan Ziadeh, a prominent Syrian dissident who was denied political asylum last year, but has been allowed to remain in the United States under the program. “I have a death sentence from the Assad government, I’m on the top of their list.”

He added, “It’s common sense actually to do the right thing, to renew T.P.S.”

It was not clear that Syrians could be deported from the United States, even if immigration officials tried, said Robert S. Ford, an American ambassador to Syria during the Obama administration who is now a senior fellow at the Middle East Institute. There is no direct air service between the two countries, and neither country maintains an official diplomatic presence in the other, which may complicate any efforts to obtain the documentation required before anyone can be deported.