WASHINGTON — Saudi Arabia announced on Wednesday night that it had launched a military campaign in Yemen, the beginning of what a Saudi official said was an offensive to restore a Yemeni government that had collapsed after rebel forces took control of large swaths of the country.
The air campaign began as the internal conflict in Yemen showed signs of degenerating into a proxy war between regional powers. The Saudi announcement came during a rare news conference in Washington by Adel al-Jubeir, the kingdom’s ambassador to the United States.
Mr. Jubeir said that the Saudis were part of a coalition of approximately 10 nations determined to blunt the advance of Shiite Houthi rebels, who have overrun Yemen’s capital and forced the American-backed government into a full retreat.
“We will do whatever it takes to protect the legitimate government of Yemen,” said Mr. Jubeir, who spoke to reporters shortly after the air campaign had begun.
Mr. Jubeir did not name the other countries involved in the military campaign, but said the coalition included other Persian Gulf nations. He said that American military forces were not involved in the airstrikes, but that the Saudis had consulted “very closely” with the Obama administration before launching the offensive.
The Saudi-led campaign began 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.
Yemen shares a long border with Saudi Arabia, a major American ally, and the Saudis had been reported to be massing forces on the Yemen frontier as Mr. Hadi’s last redoubt in Aden looked increasingly imperiled.
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. In 2011, the Central Intelligence Agency built a base in the southern desert of Saudi Arabia to launch drone strikes in Yemen.
Along with Syria, Iraq and Libya, Yemen is now the fourth state to veer toward political disintegration in the aftermath of the Arab Spring revolts, which first erupted four years ago.
By Wednesday morning, Houthi forces had seized Al Anad air base, which until recently had been used by American counterterrorism forces, about 35 miles from Mr. Hadi’s refuge in Aden, the country’s second-largest city.
A television network under Houthi control said the rebel forces had found the base empty and looted, and had captured two senior officers loyal to Mr. Hadi, including his defense minister. A security official of Mr. Hadi’s government confirmed the loss of Al Anad.
Yemeni Air Force planes under Houthi control also struck targets near the president’s Aden home, and his supporters returned fire with antiaircraft guns. The state television network, also controlled by Houthis, announced a $100,000 bounty for Mr. Hadi’s arrest as rumors about his whereabouts swirled. By nightfall, there were reports that Houthi forces were fighting around the Aden airport, on the outskirts of the city.
Shortly after the Saudi strikes began, witnesses in Sana, the capital, reported that bombing had begun at the airport and that electrical power had been cut.
A spokeswoman for the National Security Council said Wednesday night that the United States was providing intelligence and logistical support for the campaign in Yemen, and that President Obama had authorized a “joint planning cell” with Saudi Arabia to coordinate American support for the military offensive.
Mr. Hadi’s foreign minister reiterated his calls for intervention by Saudi Arabia, Egypt and other Arab states to stop the Houthis, stoking fears that their advance could set off a widening conflict.
The country appeared to be sliding toward a civil war as dangerous as any in the region, with elements of a sectarian feud, a regional proxy conflict, the attempted return of an ousted authoritarian and the expansion of anti-Western extremist groups like Al Qaeda and the Islamic State that are eager to capitalize on the chaos.
The Houthis, a minority religious group from northern Yemen, practice a variant of Shiite Islam and receive support from Iran. But they are also collaborating with Yemeni security forces still loyal to former President Ali Abdullah Saleh, the longtime strongman who was pushed from power amid the Arab Spring uprising but now appears to be orchestrating a comeback in alliance with the Houthis.
With Mr. Saleh’s help, the Houthis now control most of the Yemeni military, including its air force.
That has given them a decisive advantage over Mr. Hadi’s forces, as their seizure of the air base on Wednesday made clear.