WASHINGTON — Defense Department officials said late Wednesday that United States airstrikes and Kurdish fighters had broken the Islamic militants’ siege of Mount Sinjar, allowing thousands of the Yazidis trapped there to escape.

An initial report from about a dozen Marines and Special Operations forces who arrived on Tuesday and spent 24 hours on the northern Iraqi mountain said that “the situation is much more manageable,” a senior Defense official said in an interview.

Defense Secretary Chuck Hagel, speaking to reporters Wednesday night at Andrews Air Force Base, Md., said it was “far less likely now” that the United States would undertake a rescue mission because the assessment team reported far fewer Yazidis on the mountain than expected, and that those still there were in relatively good condition.

Rear Adm. John Kirby, the Pentagon press secretary, credited American airstrikes and humanitarian airdrops as well as efforts of the Kurdish pesh merga fighters in allowing “thousands of Yazidis to evacuate from the mountain each night over the last several days” and to escape the militants from the Islamic State in Iraq and Syria.