The airstrikes campaign was announced at the Saudi embassy in Washington by Ambassador Adel al-Jubeir in a rare news conference. Offensive military action by Saudi Arabia is also a rarity, although Saudi Arabia is a partner in the U.S.-led coalition against the Islamic State group in Syria.
There were indications that regional assistance to Yemen was extending beyond Saudi Arabia.
In a statement published by the Saudi Press Agency, Saudi Arabia, the United Arab Emirates, Kuwait, Qatar and Bahrain said they would answer a request from Yemeni President Abed Rabbo Mansour Hadi "to protect Yemen and his dear people from the aggression of the Houthi militias which were and are still a tool in the hands of foreign powers that don't stop meddling with the security and stability of brotherly Yemen." Oman, the sixth member of the Gulf Cooperation Council, didn't sign onto the statement.
The GCC is expected to meet this weekend in Saudi Arabia and Hadi is expected to attend.
Driven weeks ago from the capital by the Houthis, Hadi abandoned the country, leaving on a boat from the southern port of Aden, Yemeni security officials said. His departure came after rebel airstrikes rained down on his troops, a sign that rebels held air superiority and that Hadi's calls for an international no-fly zone had been disregarded. On the ground, the rebels were advancing toward his position.
Al-Jubeir said the Saudi air operations began at 7 p.m. EDT, and noted he "had never seen militias with air power," a reference to Iranian deliveries of weaponry to the Houthis.
Loud, house-shaking explosions could be heard in the Yemen capital of Sanaa and fire and smoke could be seen in the night sky, according to an Associated Press correspondent whose home is near the military airbase in the capital.
Al-Jubeir said the Houthis "have always chosen the path of violence." He declined to say whether the Saudi campaign involved U.S. intelligence assistance.
He sadid the Saudis "will do anything necessary" to protect the people of Yemen and "the legitimate government of Yemen."
Hadi's departure illustrated how one of the most important American counterterrorism efforts has disintegrated, leaving the country wide open for what could be a deeply destabilizing proxy war between Saudi Arabia and Iran. The Saudi announcement only reinforced that notion.
Three years ago, American officials hailed Hadi's ascension to power in a U.S.-brokered deal that ended the longtime rule of Ali Abdullah Saleh during the political upheaval of the Arab Spring. Just a few months ago, President Barack Obama was still calling Yemen a counterterrorism success story, even as the CIA warned that Iranian-backed Houthi rebels were growing restive in the north of the country.
Now, U.S. officials acknowledge their efforts against Yemen's dangerous al-Qaida affiliate are seriously hampered, with the American embassy closed and the last U.S. troops evacuated from the country over the weekend. Although the Houthis have seized control of much of the country and are avowed enemies of al-Qaida, they can't project power against the militants the way the Hadi government could with American support, officials say. Deeply anti-American, the Houthis have rejected U.S. overtures, officials say.
Al-Qaida in the Arabian Peninsula, or AQAP, is considered the terror group most dangerous to the U.S. because it successfully placed three bombs on U.S.-bound airlines, although none exploded. The chaos in Yemen will give the group breathing space, American officials acknowledge.
Beyond terrorism, the latest developments in Yemen have worrisome implications for a Middle East already wracked by Sunni-Shia conflict, experts say. Before the airstrikes began, Saudi Arabia bolstered its troop presence along its border with Yemen.
"This is all about Sunni vs. Shia, Saudi vs. Iran," said Michael Lewis, professor at Ohio Northern University College of Law and a former Navy fighter pilot who watches Yemen closely. The U.S., he said, "can't be a disinterested observer. Nobody's going to buy that. What we needed to do was pick a side."
But the U.S. had made no move to protect the Hadi government as the Houthis advanced, and American officials gave no indication Wednesday that their stance of neutrality had changed. Asked whether the U.S. military had considered trying to rescue Hadi, a senior American official who declined to be quoted answered: "The tinderbox in Yemen is most complicated because of the geopolitics at stake. The U.S., Saudis, Iranians, Houthis, Yemenis, AQAP, ISIL and AQ have equities in the situation and factor into any decision the U.S. makes or doesn't make."
As late as Monday, officials insisted the U.S. was still working with Hadi's government, despite the fact that the president had been forced out of the capital and the parliament dissolved.
"There continues to be ongoing security cooperation between the United States and the national security infrastructure of the Hadi government," White House spokesman Josh Earnest said.
Associated Press writers Ahmed al-Haj in Sanaa, Yemen, Sarah el-Deeb in Cairo and Lolita C. Baldor and Julie Pace in Washington contributed to this report.