WASHINGTON — Saudi Arabia announced on Wednesday night that it had begun military operations in Yemen, launching airstrikes in coordination with a coalition of 10 nations.

The strikes came as Yemen was hurtling closer to civil war after months of turmoil, as fighters and army units allied with the Houthi movement threatened to overrun the southern port of Aden where the besieged president, Abdu Rabbu Mansour Hadi, has gone into hiding.

The rapid advances by the president’s opponents included the seizure of a military air base and an aerial assault on his home. There were unconfirmed reports that the president had fled the country by boat for Djibouti, the tiny Horn of Africa nation across the Gulf of Aden.

The region’s most impoverished country, Yemen has been a central theater of the American fight against Al Qaeda, and its possible collapse presents complex challenges to the Obama administration as it struggles to deal with instability and radical extremism in the Middle East.