Saudi-led coalition jets have pounded Houthi targets for the second day in Yemen, including the Shi'ite rebels' stronghold of Saada in the north of the country.

Airstrikes hit the capital Sanaa and key military installations held by the Houthis, including the al-Anad Air Base in Aden in the south and the Tariq Air Base in the country's third city of Taiz. Aerial bombings also targeted a reception camp run by commanders loyal to ousted president Ali Abdullah Saleh, who backs the Shi'ite rebels.

Amnesty International said at least six children were among the 25 people killed in air strikes in Sanaa on Thursday.

The offensive, led by Saudi Arabia but supported by more than 10 countries, five of which are members of the Gulf Cooperation Council, started after Yemeni president Abd-Rabbu Hadi pleaded for help when the Houthis closed in on his government in exile in Aden.

A spokesman for the coalition said the military operation would continue "as long as necessary". There are no plans for the deployment of ground forces but Saudi Arabia amassed 150,000 troops on its porous border with Yemen.

Adel al-Jubair, Saudi ambassador to the US, said the first wave of airstrikes over targets in Yemen "went extremely well and with no collateral damage".

Hadi arrived in Riyadh on Thursday and plans to continue its journey to Egypt to take part in a two-day Arab League summit at the weekend as Yemen's "legitimate" president. Iran called the Saudi offensive a "dangerous step" that will worsen the crisis.

UAE, Bahrain, Kuwait, Qatar, Jordan, Morocco and Sudan are sending aircraft to Yemen. Egypt, Jordan, Sudan and Pakistan said they were ready to take part in any ground offensive targeting the Houthis.

The US said it was providing "logistical and intelligence support".