Defense officials said late Wednesday that United States airstrikes and Kurdish fighters had broken the siege on Mount Sinjar, allowing thousands of Yazidis trapped there to escape. An initial report from about a dozen Marines and Special Operations forces who spent the last 24 hours on the mountain said that “the situation is much more manageable,” a senior defense official said in an interview. “A rescue effort now is much more unlikely,” the official said. Defense officials could not say how many Yazidis remained on the mountain, but Defense Secretary Chuck Hagel was expected to make an statement later Wednesday night. The announcement came after American military advisers landed on Mount Sinjar early Wednesday to begin assessing how to organize the evacuation. The United States had said it would consider using American ground troops to assist in the rescue if recommended by the military team.