The airport of the capital Sanaa and other targets in Yemen came under air attack early Thursday, as Saudi Arabia announced that it had launched a military campaign tp defend the government of President Abdu-Rabbu Mansour Hadi. Houthi militias in alliance with forces loyal to former President Ali Abdullah Saleh lost a key air base early Thursday, a day after seizing it, officials said.

The Saudi airstrikes targeted the presidential palace and the police and special forces headquarters in the capital, Sanaa where loud, house-shaking explosions resonated in the night.

Strikes were also reported on targets in the Malaheez and Hafr Sufyan regions of Saada, a main Houthi stronghold on border with Saudi Arabia, AFP reported.

A witness told Reuters that four or five houses near the Sanaa airport had been damaged. Rescue workers put the death toll from the air strikes at 13, including a doctor who had been pulled from the rubble of a damaged clinic.

Civil defense sources told AFP news agency that at least 17 civilians were killed in Sanaa during the overnight offensive.

The Houthis confirmed in a statement to reporters that Saudi jets hit a military base in Sanaa, known as al-Duleimi. They said they fired anti-aircraft missiles in response.

Sources confirmed to Al Jazeera the killing of three Houthi military commanders: Abdel Khaleq Badereddine Al-Houthi, Yousef Al-Madani and Yousef Al-Fishi. A fourth, Mohammed Ali Al-Houthi, was injured.

Houthi-run al-Masirah television reported that the Saudi-led air strikes had hit a residential neighbourhood north of Sanaa and caused dozens of casualties. It also urged medical personnel to report to hospitals in Sanaa immediately.

Reports of casualties could not be verified.

Saudi Arabia's ambassador to the U.S., Adel A. Al-Jubeir, announced the campaign, which began at 7 p.m. EDT, saying "the operation is to defend and support the legitimate government of Yemen and prevent the radical Houthi movement from taking over the country." He also said Riyadh had held "productive and fruitful" consultations with the United States over the intervention, and was appreciative of the support offered by Washington.

He declined to say whether the Saudi campaign involved U.S. intelligence assistance, and a White House spokeswoman referred questions to the Saudis. However, several hours after the operation began, the White House issued a statement saying President Barack Obama had authorized logistical and intelligence assitance.

"While U.S. forces are not taking direct military action in Yemen in support of this effort, we are establishing a Joint Planning Cell with Saudi Arabia to coordinate U.S. military and intelligence support," the White House statement said.

Saudi-owned Al-Arabiya News reported that the kingdom has deployed 100 fighter jets, 150,000 soldiers and other navy units. There was no immediate confirmation of those figures from Riyadh. Al-Arabiya also said the United Arab Emirates was sending 30 warplanes to join the operation, along with 15 each from Bahrain and Kuwait, 10 from Qatar, six each from Jordan and Morocco and three from Sudan.

Other regional players were involved in the Saudi operation, according to The Associated Press: The United Arab Emirates, Kuwait, Qatar and Bahrain joined Saudi Arabia in a statement published by the Saudi Press Agency, saying they would answer a request from 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.

Egypt also announced political and military support. "There is coordination ongoing now with Saudi Arabia and the brotherly gulf countries about preparations to participate with an Egyptian air and naval forces and ground troops if necessary," it said in a statement carried by the state news agency.

Pakistan, Jordan, Morocco and Sudan were also joining the operation, the Saudi Press Agency reported Thursday.Speaking to Al Jazeera from Sanaa, Houthi spokesman Mohammed Al Bukhaiti called the military action a declaration of war on Yemen, adding that reports alledging that Houthi leader Mohamed Ali Al Houthi had been injured were false.

Iranian state media called the military operation a "US-backed aggression." State television broadcast footage of some of the damage, showing dozens of bodies and some wounded people, saying "many Yemeni citizens were killed in the US-backed aggressions in Yemen."

And amid the chaos currently unfolding in Yemen, the Los Angeles Times reports that Iran-backed Houthi forces had looted secret files held by the Yemeni security forces containing details of U.S. counterterror operations in the country.

After taking Al-Anad air base, outside Aden, the Shia Houthis and their allies advanced to within 25 miles of Aden, where Hadi has been holed up since he fled a Houthi takeover of the capital, Sanaa, last month.

One of his aides said Hadi was "in high spirits" after the operation began. An Aden official later said Hadi loyalists had retaken the southern city's airport after heavy clashes.

Contradicting earlier reports from the Associated Press that the president had fled the country, his office manager, Mohamed Marem, told Al Jazeera late Wednesday that Hadi was in a safe place in Aden, "closely following" the violence at his doorstep.

But he said forces loyal to Hadi were in dire need of weapons and called on the U.S. to take a "stronger position" against the Houthi rebellion. He also said Hadi on Wednesday formally requested a military intervention from the Arab League, a day after asking the same of the Saudi-led Gulf Cooperation Council (GCC).

Hadi’s foreign minister, Riad Yassin, had told Al Jazeera on Tuesday that Hadi's request was accepted and that “arrangements are taking place,” but did not elaborate. The GCC maintains the Peninsula Shield Force, a rarely used army of 30,000 that was last deployed to quell Bahrain’s Shia-led uprising in 2011.

Residents in Aden said Wednesday that unidentified warplanes fired missiles at the neighborhood surrounding Hadi's compound. Anti-aircraft batteries returned fire, and pro-Hadi militiamen and tribal gunmen were out in force throughout the city. Parents took schoolchildren home, and public sector employees obeyed orders to leave work.

“The situation is so tense, [and there are] very loud explosions heard,” Morad Abdo, a journalist in Aden, told Al Jazeera on Wednesday evening. “I expect during the upcoming few hours that [Houthi-allied forces] … will invade Aden,” he said.

"The war is imminent, and there is no escape from it," 21-year-old Mohammed Ahmed said outside a security compound in the city’s Khor Maksar district, where hundreds of young men have been signing up to fight the advancing Shia fighters. "And we are ready for it.”

Yemen's apparent slide toward civil war has made the country a crucial front in mostly Sunni Saudi Arabia's rivalry with Shia Iran. Riyadh blames the ousting of its ally Hadi on Iran, which it says has stirred sectarian strife by backing the Houthis. Other Sunni Arab monarchies in the region have similarly condemned the Shia Houthi takeover as a Tehran-backed coup, though the Iranians have denied involvement.

As fighting has intensified in Aden province this week, U.S. officials said Saudi Arabia was moving heavy military equipment, including artillery, to areas near its border with Yemen, raising the risk that Riyadh could be drawn into the conflict more directly.

Saudi Arabia has said its military movements, which were first reported Tuesday night, are defensive in nature.