Warplanes from a Saudi-led coalition are bombing Yemen for a third night, targeting Shia Houthi rebels as they advance in and around the port of Aden.
The rebel advance has sparked street battles with forces loyal to President Abdrabbuh Mansour Hadi, who fled Aden and is now at an Arab League summit.
Former President Ali Abdullah Saleh – whose supporters are fighting alongside the rebels – has called for a truce.
But Saudi Arabia has vowed to defend Mr Hadi’s government from the rebels.
The Sunni kingdom has accused its regional rival, Shia-led Iran, of backing the Houthis.
It has mobilised a coalition, including Gulf Arab states and Egypt, to roll back the rebels’ advance in a mission codenamed Operation Decisive Storm.
On Friday, warplanes carried out raids on Yemen’s rebel-held capital, Sanaa, as well as on the Houthis’ northern heartland of Saada.
The raids targeted arms depots, military bases as well as buildings used by Houthi leaders, residents and military officials said.
Since the air campaign began, at least 39 civilians – including six children under the age of 10 – have been killed, Yemen health ministry officials say.
A resident of Sanaa, Mohammed al-Jabahi, told AFP news agency that his family had spent the night in fear.
“Whenever a plane flies over our home and is met by anti-aircraft gunfire, my three children run to a corner and start screaming and crying,” he said.
People have been fleeing the capital, with long queues at petrol stations, and many shops and firms have shut.
Meanwhile, the rebels have gained their first foothold on the Arabian Sea coast by seizing the town of Shaqra, 100 km (60 miles) east of Aden, residents told Reuters news agency.
The move gives the rebels control over all the land routes to Aden, a stronghold of forces loyal to Mr Hadi.
Mr Hadi took refuge in the second city of Aden last month after fleeing Sanaa, where he had been under house arrest since the rebels took full control of the capital in January.
On Thursday he fled to Saudi Arabia, from where he travelled to Egypt on Friday to attend an Arab League summit, likely to be dominated by the crisis in Yemen.
In an interview with the BBC at the summit on Thursday night, Yemeni Foreign Minister Riyadh Yassin said no-one was happy about the intervention by the Saudi-led coalition.
But he added: “I think if they completed their mission in the coming few days or few hours it will be stopped. It is a short, sharp campaign which really we have been forced to request.”
Mr Yassin said he did not know if Arab leaders would approve a ground offensive, which coalition members have said they are prepared to launch if the raids fail to halt the rebels’ advance and force them to negotiate.
Rebel leader Abdul Malik al-Houthi has vowed not to surrender to what he called the “unjustified aggression”.
Iran has also criticised the Saudi intervention. “They have to stop,” Iranian Foreign Minister Mohammad Javad Zarif said on Friday.
“Everybody has to encourage dialogue and national reconciliation in Yemen rather than making it more difficult for Yemenis to come together.”
Meanwhile, Mr Saleh, whose supporters are fighting alongside the Houthis, has called for a ceasefire, followed by negotiations.
Yemen has been wracked by instability since the former president stepped down amid massive protests in 2012.
The country’s conflict has inflamed sectarian rivalries in the region. In Lebanon, the leader of the powerful Shia Hezbollah militia, Hassan Nasrallah, mocked the Saudi-led offensive.
“Should the region go to war because of Saudi money?” he asked. He said Iran had expanded its influence because the Saudis were “lazy, losers, and… don’t take responsibility”.
Rebel forces are still fighting Popular Resistance Committees militiamen and Sunni tribesmen loyal to President Hadi in the south.
Saudi and Egyptian warships are also believed to have been deployed to the Bab al-Mandab strait to secure the strategic passage between the Red Sea and Gulf of Aden.
On Friday, at least 21 Houthis were killed when they were ambushed about 15km (9 miles) north of Aden, witnesses told AFP. Another eight people reportedly died in clashes around Aden’s international airport.
The Houthis have said their aim is to replace Mr Hadi’s government, which they accuse of being corrupt, and to implement the outcomes of a National Dialogue that was convened after President Saleh was forced to hand over power in 2011.
The Houthis: Zaidi Shia-led rebels from the north, who seized control of Sanaa last year and have since been expanding their control
President Hadi: Fled to Saudi Arabia after rebel forces advanced on his stronghold in the southern city of Aden
Al-Qaeda in the Arabian Peninsula: Seen by the US as the most dangerous offshoot of al-Qaeda, AQAP opposes both the Houthis and President Hadi.
Islamic State: A Yemeni affiliate of IS has recently emerged, which seeks to eclipse AQAP