Saudi-led air raids drove back rebels in the last stronghold of Yemen's absent president yesterday, while al-Qaeda militants seized a major army base in the southeast of the country.

The impoverished Arabian Peninsula state has sunk further into chaos since the coalition spearheaded by Riyadh launched Operation Decisive Storm on March 26 to try to halt the advance by Shia Huthi rebels.

The turmoil has raised fears that al-Qaeda will expand its foothold in the deeply tribal country, which borders oil-rich Saudi Arabia and lies near key shipping routes.

On Friday the Sunni extremists captured the regional army headquarters in Mukalla, capital of the southeastern province of Hadramawt, with no resistance, a military official said.

The militants now control nearly all of the city, where they stormed a jail and freed 300 inmates a day earlier.

UN aid chief Valerie Amos said Thursday that 519 people had been killed and nearly 1,700 wounded in two weeks of fighting, adding she was "extremely concerned" for the safety of trapped civilians.

The conflict has sent tensions soaring between Iran and Saudi Arabia, the foremost Shia and Sunni Muslim powers in the Middle East.

Iran has angrily rejected accusations or arming the rebels, who have allied with military units loyal to former president Ali Abdullah Saleh to seize large parts of Yemen, including the capital Sanaa.

After a night of intense coalition bombardment, rebel forces withdrew from President Abedrabbo Mansour Hadi's palace in Aden, a senior official said.

They had captured the hilltop complex a day earlier in a symbolic blow to Hadi, who has fled to Saudi Arabia.

The rebel forces retreated to the nearby central district of Khor Maksar, where 12 rebels were reported dead in an overnight attack by pro-Hadi militiamen.

The coalition air dropped rifles, ammunition and communications equipment to supporters of the president in Aden battling to prevent its fall, according to a local official.

Before the latest chaos erupted, Yemen had been a key US ally in the fight against al-Qaeda, allowing Washington to carry out drone attacks on its territory.

The government's collapse forced the United States to close its embassy and withdraw US special operations forces that were helping Yemeni forces battle AQAP.

Members of al-Qaeda in the Arabian Peninsula (AQAP) launched calls from mosques in the city for "jihad against Shias", according to residents.

Meanwhile, as part of its logistical support for the Saudi-led campaign, US will provide aerial refuelling, the official said.

The United States was also delivering intelligence from surveillance satellites and aircraft to help the Saudis monitor their border and to track Huthi rebels as they push south, the official added.

Meanwhile, China said it had evacuated from Yemen 225 people from 10 different countries aboard a missile frigate, an unprecedented move that underscores its growing global reach.

The evacuees included 176 people from Pakistan, the foreign ministry said, with the remainder from Ethiopia, Singapore, Italy, Germany, Poland, Ireland, Britain, Canada and Yemen.