The bombings continued into Thursday, with officials saying warplanes were targeting the Houthis' ability to launch their own attacks by air.  The rebels took control of an airbase in southern Yemen on Wednesday and later bombed the presidential complex in the nearby city of Aden.

　　Officials said Hadi had fled the site, and his exact whereabouts are not publicly known.

　　Saudi Ambassador to the U.S. Adel al-Jubeir told reporters late Wednesday that the Saudis will do "anything necessary" to protect the people of Yemen and its "legitimate government."

　　Saudi Arabia Launches Airstrikes in Yemen

　　The White House said the United States is coordinating with the Saudi-led military coalition and providing "logistical and intelligence support," but not taking direct military action.

　　"We strongly urge the Houthis to halt immediately their destabilizing military actions and return to negotiations as part of the political dialogue," National Security Council spokeswoman Bernadette Meehan said in a statement.  "The international community has spoken clearly through the U.N. Security Council and in other fora that the violent takeover of Yemen by an armed faction is unacceptable and that a legitimate political transition -- long south by the Yemeni people -- can be accomplished only through political negotiations and a consensus government among all of the parties."

　　In addition to taking over the Al-Anad air base, forces loyal to the Houthis also captured the international airport in Aden on Wednesday as rebel fighters advanced on the city where Hadi had been taking refuge since leaving the capital last month.

　　Yemen has sunk into violence and chaos since a popular uprising ousted longtime strongman President Ali Abdullah Saleh in 2012.

　　The Houthis, widely believed to be backed by Iran, have taken over the capital and have battled Sunnis and other tribes as they try to expand their authority throughout the rest of the country.

　　President Hadi has sent a letter to the U.N. Security Council, saying "all our efforts for peaceful settlement have encountered absolute rejection by the Houthis, who continue their aggression to subdue the rest of the regions out of their control."

　　"The Yemeni people have never faced such heinous aggression, which is against the basic principles of Islamic and international norms and conventions," the president wrote.

　　Analysts say Saudi Arabia's move to intervene could escalate the conflict into a regional battle.

　　“Iran is already helping the Houthis, and I expect Iran to ramp up support for its Yemen proxy,” said Max Abrahms, a professor of political science at Northeastern University and member at the Council of Foreign Relations. “Yemen is a sectarian tinderbox that will likely explode with international geopolitical repercussions.”

　　A sectarian civil war with external intervention will have repercussions, Abrahms said.

　　“The rational course of action is for Saudi Arabia and other leading Sunni countries to halt Iranian influence around the world,” he said. “Not because of the differences between Sunni and Shia Islam, but because of competing national interests and the geopolitical stakes.”

　　Earlier, some analysts predicted that it was not likely that Saudi Arabia would do much more than shore up its border.

　　“Although the Saudis have used an increasingly more dire - perhaps even bellicose - tone in their public statements about the violence in Yemen and what must be done to stop it, a military intervention would be incredibly risky,” said Fahad Nazer, a former political analyst at the Saudi Embassy in Washington.

　　Nazer said Sauda Arabia would need the support of other countries in the region. It was unclear if that happened Wednesday.

　　Other countries in the region may not have much appetite to go to war as the crisis does not impact them as strongly as it does the Saudi Kingdom, Nazer said.

　　“It is also important to note that the Saudis and other GCC states are already taking part in the US-led strikes against ISIS strongholds in Syria,” Nazer said. “Any additional military commitments could stretch their military forces thin.”

　　Perhaps most importantly, the Saudis face more than one adversary in Yemen, which is home to al Qaida in the Arabian Peninsula [AQAP] and, if their claims are true, the so-called Islamic State, which claimed responsibility for attacks on two mosques in the capital Sana’a a week ago.

　　With the air strikes, Saudi Arabia could find itself in a bind, analysts say.

　　“They would find it difficult to bomb Houthi rebels and let AQAP roam wild in the south,” Nazer said. “Targeting the Houthis and sparing AQAP would enhance the latter's position on the ground. Some Western and Middle East critics of Saudi foreign policy would likely frame such an attack as tacit support of AQAP.”

　　In the long term, Saudi Arabia is reported to be planning to build new military facilities, including a naval base at the Red Sea port of Jazan.