SANAA, Yemen — Saudi Arabia launched airstrikes early Thursday in neighboring Yemen, heading a coalition of Arab nations in an effort to dislodge Houthi rebels sweeping through that country.

The strikes were a startling turn of events that came as the Houthis, in control of Yemen’s capital for months, barreled south toward the coastal city of Aden, seizing an air base along the way that was evacuated by U.S. Special Operations forces­ last week.

President Abed Rabbo Mansour Hadi, who had taken refuge in Aden after fleeing Sanaa, the capital, was said to have escaped. His whereabouts were unknown.