Warplanes from a coalition led by Saudi Arabia have continued bombing Houthi targets in Yemen for a second day, including the Shia rebel group's stronghold of Saada, as embattled President Abd-Rabbu Mansour Hadi arrived in the Saudi capital Riyadh.
A spokesman of the coalition said on Thursday that the military operation against the Shia Houthi rebels would continue "as long as necessary". Brigadier Ahmed 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 Riyadh on Thursday, with officials saying he would continue his journey 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 has been holed up since fleeing the rebel-controlled capital last month.
Saudi Arabia began the air campaign on Thursday night, saying it had assembled a coalition of more than 10 countries, five of them members of the six-nation Gulf Cooperation Council.
The Saudi ambassador to the US, Adel al-Jubeir, said the coalition stood ready to do "whatever it takes" to protect Hadi's government.
Explosions have been heard in the capital, Sanaa, which has been under Houthi control since September. The Shia rebels seized power in a coup last month.
Thousands protest
Al Jazeera received reports that air strikes targeted a reception camp of new recruits joining militias loyal to ousted President Ali Abdullah Saleh - who backs the Houthis - west of Sanaa.
Eyewitnesses also reported air strikes and loud blasts in Saada near the Saudi border, where a military unit was the target.
Al Jazeera also learned that the air strikes hit al-Anad Air Base in Aden in the south and the Tariq Air Base in the country's third city of Taiz.
Rights group Amnesty International said at least six children were among 25 people killed in the air strikes in the capital on Thursday. Earlier, Houthi sources said at least 18 people had been killed in the bombardment.
Thousands of protesters gathered in Sanaa against the air raids.
In a statement following the strikes, the White House said that the US was coordinating military and intelligence support with the Saudis but not taking part directly in the raids.
Jeff Rathke, a US State Department spokesman, said on Thursday that the US government "understands the concerns" of the Saudis and is "supportive of their effort".
Iranian condemnation
Houthi leader Abdel-Malek al-Houthi, in a televised speech, described the Saudi-led operation as a "despicable aggression".
"What do they expect us to do, surrender, announce our defeat and act like cowards? Absolutely not. This is not how the honorable Yemeni people think. We will fight back. All 24 million Yemenis will stand united and face that despicable aggression," al-Houthi said.
Ousted president Saleh also called on the Houthis to stop attacking Aden, even as he denounced the Saudi air strikes inside his country.
Iran, which is accused of backing the Houthis but denies the charge, has condemned the intervention as "a dangerous step" that violated "international responsibilities and national sovereignty".
President Hassan Rouhani said it amounted to "military aggression" and "condemned all military intervention in the internal affairs of independent nations".
Iran's Foreign Minister Mohammad Javad Zarif, speaking from Switzerland where he is attending talks on his country's nuclear programme, warned that air strikes would lead only to greater loss of life .
Recep Tayyip Erdogan, Turkey's president, said that Iran had been trying to dominate the Middle East.
"It is really not possible to tolerate this. Iran has to understand," he said, adding that Tehran should withdraw any forces it had in Yemen as well as from Syria and Iraq.
Saudi television said the kingdom had deployed 100 fighter jets to the operation, while the United Arab Emirates had committed 30, Kuwait 15 each and Qatar 10. Bahrain said it had committed 12 fighter jets. All five are members of the GCC. There was no mention of Oman, the sixth GCC member.
Saudi Arabia had also mobilised 150,000 troops near the border.
Riyadh said it was boosting security on its borders and across the kingdom, including at the OPEC kingpin's crucial oil facilities.