Warplanes from a Saudi-led coalition bombed Houthi targets in Yemen for a second night, including the Shia rebel group's stronghold of Saada, as embattled President Abd-Rabbu Mansour Hadi arrived in Riyadh.
Brigadier Ahmed al-Asiri, a spokesman for the coalition said on Thursday that the military operation against the Shia Houthi rebels will continue "as long as necessary."
Al-Asiri also said that "at the moment" there are no plans for the deployment of ground forces, but troops are "ready for all the circumstances."
President Hadi arrived in the Saudi capital on Thursday, with officials saying he was on his way to Egypt to take part in a two-day Arab League summit at the weekend.
That was the first confirmation of Hadi's whereabouts since the rebels began advancing this week on the main southern city of Aden, where the president had been holed up since fleeing the rebel-controlled capital last month.
Rights group Amnesty International said at least six children were among 25 people killed in the air strikes in the capital on Thursday. Earlier, Yemen's health ministry had said at least 18 civilians had died and 24 had been wounded.
Saudi Arabia carried out airstrikes in Yemen on Thursday after announcing a broad regional coalition to oust Shia rebels who have forced the country's embattled president to flee.
Some of the raids hit positions in the country's capital, Sanaa, and flattened a number of homes near the international airport.
Saudi Arabia's regional rival Iran condemned the move, warning against "short-sighted games" by outside countries that would only lead to "bloodshed" and further loss of life.
The Saudi airstrikes came hours after Hadi, a close U.S. ally, fled Yemen by sea after rebels pushed their way toward the southern port city of Aden, where he had taken refuge.
Earlier Thursday, the official Saudi Press Agency reported that Hadi had arrived in the Saudi Arabian capital, Riyadh. His whereabouts since he fled had previously been unknown.
Saudi-owned Al-Arabiya News reported that the kingdom had deployed 100 fighter jets, 150,000 soldiers and other navy units in Operation Decisive Storm.
A ground offensive may be needed to restore order in Yemen, a Saudi source familiar with defense matters said Thursday, hours after the kingdom began spearheading the airstrikes against Iranian-allied Houthis.
Four Egyptian naval vessels have crossed the Suez Canal en route to Yemen to secure the Gulf of Aden, maritime sources at the Suez Canal said. The sources said they expected the vessels to reach the Red Sea by Thursday evening.
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 Houthi takeover as a Tehran-backed coup, though the Iranians have denied involvement.
Houthis were calling on their supporters to protest in the streets of Sanaa on Thursday afternoon, Yemen's Houthi-controlled state news agency SABA reported, after three Houthi military commanders were killed in the air strikes.
TV stations affiliated with the rebels and their ally, former President Ali Abdullah Saleh, showed the aftermath of the strikes Thursday morning in what appeared to be a residential area.
Yemen Today, a TV station affiliated with Saleh, showed hundreds of residents congregating around a number of flattened houses, some chanting "Death to Al-Saud," in reference to the kingdom's royal family. The civilians were sifting through the rubble, pulling out mattresses, bricks and shrapnel.
Local resident Ahmed al-Sumaini said an entire alley close to the airport was wiped out in the strikes overnight. He said people ran out from their homes in the middle of the night. "This was a surprise. I was asleep, and I was jolted out of my bed," he said, waving a piece of shrapnel.
In addition to the airport, targets included the camp of U.S.-trained Yemeni special forces, which is controlled by generals loyal to Saleh. Yemeni security officials said the targets also included a missile base in Sanaa that was controlled by the Houthis earlier this year. One of the security officials said the strikes also targeted the fuel depot at the base.
The Houthis said in a statement that Saudi jets hit the military base, known as al-Duleimi, and that they responded with anti-aircraft missiles. The strikes also hit the Annad air base in the southern Lahj province. About 100 U.S. military advisers withdrew over the weekend from base, where they had been leading a drone campaign against Al-Qaeda in the Arabian Peninsula, or AQAP.
The crumbling of Hadi's government is a blow to Washington's counterterrorism strategy against AQAP, considered to be the most powerful in the terrorist network.
“The United States strongly condemns ongoing military actions taken by the Houthis against the elected government of Yemen. These actions have caused widespread instability and chaos that threaten the safety and well-being of all Yemeni citizens,” National Security Council spokeswoman Bernadette Meehan said in a statement.
The White House said late Wednesday that the U.S. was coordinating military and intelligence support with the Saudis but not taking part directly in the strikes.
The airstrikes, which had the support of nine other countries, drew a strong reaction from Iran, which called the operation an "invasion" and a "dangerous step" that will worsen the crisis in the country.
Iranian Foreign Minister Mohammad Javad Zarif said: "Military action from outside of Yemen against its territorial integrity and its people will have no other result than more bloodshed and more death."
Calling for "urgent dialogue" among Yemeni factions, he added :"We have always warned countries from the region and the West to be careful and not enter short-sighted games and not go in the same direction as al-Qaeda and Daesh," he added, using the Arabic acronym for the Islamic State of Iraq and the Levant (ISIL) group.
The back-and-forth between the regional heavyweights was threatening to turn impoverished Yemen into a proxy battle between the Middle East's Sunni powers and Shia-led Iran.
In the Saudi-led mission, the United Arab Emirates is participating with 30 jets, Bahrain with eight, Morocco and Jordan both with six. Sudan reportedly offered three war planes to assist the operation, Al-Arabiya reported. Jordan confirmed to Al Jazeera that it was participating in the offensive.
An Egyptian official told the AFP news agency that Egypt would also take part. Saudi Arabia said that another four Muslim countries, including Pakistan, wanted to participate in the Saudi-led military coalition. Kuwait's Defense Ministry announced it was sending three squadrons of its F-18 Super Hornet aircraft to Saudi's King Abdulaziz airbase in Dhahran to take part in the offensive.
Turkey said Thursday that it supported the Saudi-led operation. The Turkish foreign ministry said in a statement that Saudi Arabia had informed Ankara about the operation beforehand and that it believed the operation would revive legitimate state authority and prevent the risk of civil war.
Yemen now faces fragmentation, with Houthis controlling much of the north, including the capital, and several southern provinces. In recent days, they took the third-largest city, Taiz, as well as much of the province of Lahj, both just to the north of Aden.
The Houthis are backed by Saleh, the autocrat who ruled Yemen for three decades until he was removed amid a 2011 Arab Spring uprising. Some of the best-equipped and trained military and security units remained loyal to Saleh and they have helped the Houthis in their rapid advance.
Hadi left Sanaa for Aden earlier this month after escaping house arrest under the Houthis, who overran the capital six months ago. In Aden, he had sought to make a last stand, claiming it as the temporary seat of what remained of his government, backed by allied militias and loyal army units.
With Houthis and Saleh forces closing in on multiple fronts, Hadi and his aides left Aden Wednesday on two boats in the Gulf of Aden, security and port officials said.
Arab leaders are meeting in Egypt this weekend for a previously planned summit. It is unclear if Hadi will join them.
Al Jazeera and wire services