WASHINGTON — President Trump has decided to slash the American refugee program by almost half, greatly dimming the United States’ role in accepting persecuted refugees from most parts of the world, the State Department announced Thursday.

The administration said it would accept 18,000 refugees during the next 12 months, down from the current limit of 30,000 and a fraction of the 110,000 President Barack Obama said should be allowed into the United States in 2016, his final year in office.

Even that low figure may overstate the number of slots that could be open for unanticipated crises, since many of the openings have been allocated. The Trump administration will reserve 4,000 refugee slots for Iraqis who worked with the United States military, 1,500 for people from Central America and 5,000 for people persecuted for their religion, senior administration officials said. The additional 7,500 slots are for those who are seeking family unification and have been cleared for resettlement.

That will eliminate many opportunities for people fleeing war and persecution throughout the world to resettle in the United States, which until Mr. Trump took office was the world’s leading destination for refugees.