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, a day after 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”.

The Houthis took control of Sanaa in September, and have since taken control of territory in several provinces. Last month, they took control of government and the president fled to the southern city of Aden. Officials in Riyadh said he left the city under Saudi protection on Thursday, after rebels began advancing this week on Aden.

They said Hadi would continue his journey to Egypt to take part in a two-day Arab League summit starting on Saturday.

Saudi Arabia began the air campaign in the early hours of Thursday, saying it had assembled a coalition of more than 10 countries, five of them members of the six-nation Gulf Cooperation Council.

Rebel positions in at least six provinces have since been hit, including air bases in Aden and Taiz. The Houthi stronghold of Saada has also been targeted.

Civilians killed

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 while in Taiz, thousands came out supporting Hadi and Saudi Arabia.

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 that the US government “understands the concerns” of the Saudis and is “supportive of their effort”.

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 honourable Yemeni people think. We will fight back. All 24 million Yemenis will stand united and face that despicable aggression,” al-Houthi said.

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 country’s crucial oil facilities. Al Jazeera