SANA'A, Yemen - Saudi Arabia and its Persian Gulf allies launched air attacks early Thursday in neighboring Yemen, after Shiite rebels believed backed by Iran swept toward that country's second-largest city and forced the president to flee.

The Saudi ambassador to the United States, Adel al-Jubeir, made the announcement on Wednesday evening in Washington.

He said the operation began at 7 p.m. Eastern U.S. time in order "to prevent Yemen from falling into the hands of the Houthis" - the rebels.

The operation was an extraordinary development that could plunge Yemen into an all-out war. The ambassador said military forces from several countries were preparing to join the operation, apparently referring to Saudi Arabia's Gulf allies. He said the airstrikes targeted sites around the country, including in its capital, Sana'a. The Saudi government had consulted "very closely and very intensely with our partners, in particular the United States," he said.

Jubeir said the operation would continue until President Abed Rabbo Mansour Hadi was returned to office. "The operations are limited to defending the government and preventing its collapse," he said.

The Gulf countries acted as Aden was close to being seized by the rebels. That would give them control of both Sanaa and the country's main sea gateway.

That could have marked the end of Hadi's bid to stay in power. He had been a key Saudi ally and had worked closely with the U.S. government to fight al-Qaeda.

Yemen's branch of al-Qaeda holds patches of the country and views the Houthis as foes in the competition for influence and Yemen's modest oil wealth.

On a broader level, Yemen represents a potential proxy battlefield for Shiite power Iran and the Sunni Gulf Arab states allied with Washington, which had counted on Hadi as a partner in coordinating drone strikes against al-Qaeda.

In recent days, neighboring Saudi Arabia has massed troops and dispatched tanks to its border with Yemen, a sign of its intense unease at the idea of Iranian-backed rebels taking control of the country.

Saudi Arabia launched airstrikes against Houthi rebels near the countries' border in 2009, after protesting that its border guards were fired on.

But any ground intervention would require a long and difficult trip through the heart of Houthi-held territory to reach Aden. And it appeared unlikely that Saudi troops could roll back Houthi control of large parts of Yemen.