Aid agencies warned Monday of a growing humanitarian crisis including food shortages in Yemen as Saudi-led warplanes hit rebel positions for a third week and rival forces clashed.
In Riyadh, Yemen's Prime Minister Khaled Bahah was sworn in as vice president at the country's embassy in front of exiled President Abedrabbo Mansour Hadi, a day after his appointment, in a move welcomed by Yemen's Gulf neighbours.
Yemen's main southern city of Aden saw the heaviest fighting overnight, with medics and military forces saying at least 30 people were killed in clashes between rebels and supporters of Hadi.
Residents said warplanes from a Saudi-led coalition of countries pounded the rebel-held presidential complex and other positions in Aden, Hadi's last refuge before he fled to neighbouring Saudi Arabia as the air war began on March 26.
The Houthi Shia rebels, who have joined with forces loyal to ex-president Ali Abdullah Saleh, advanced on Aden last month after seizing control of the capital Sanaa last year.
Saudi Arabia has accused Iran, the main Shia power, of backing the rebel offensive and rallied support from regional allies for the coalition.
Western powers have also backed Hadi as Yemen's legitimate ruler, while the United Nations has called for a resumption of UN-brokered talks aimed at bringing stability to the deeply divided country.
Heavy fighting in Aden on Sunday night left at least 13 civilians, 11 rebels and six pro-Hadi fighters dead, medical and military sources said.
Humanitarian groups have struggled to bring aid into the country and said Monday the situation in Aden was deteriorating rapidly.
"Shops are closed. We have a problem of food," said Marie-Elisabeth Ingres, the Yemen representative of Doctors without Borders (MSF).
Metaz al-Maisuri, an activist living in Aden, said basic services had stopped and there had been a "mass exodus" of civilians from the city.
"Schools, universities and all public and private facilities have been shut due" to the violence, he told AFP.
"Residents' lives have become very difficult and complicated... They can no longer obtain the food they need," he said.
"We are unable to leave our houses to buy what we need because of the Houthi snipers," said Adwaa Mubarak, a 48-year-old woman in Aden.
"Our living conditions are bad. Bakeries are shut," she said, alleging that several people have been shot dead as they queued to buy bread.
UN Secretary General Ban Ki-moon warned from Doha on Sunday that the escalation in fighting was leading to a huge humanitarian crisis in Yemen as "civilian casualties are mounting and public infrastructure is being destroyed."
The UN chief said more than 600 people had been killed and 2,000 injured so far.
Aid workers said Sanaa is also suffering, as air strikes hit rebel positions in the capital and supplies dwindle.
"There is a food and water shortage. People are unable to move," said Marie Claire Feghali, the Red Cross spokeswoman in Yemen.
The International Committee of the Red Cross flew more than 35 tonnes of medical aid and equipment into Sanaa on Saturday, after the first aid deliveries Friday organised by the Red Cross and United Nations.
Thousands of foreigners have been trapped by the fighting, with the International Organization for Migration saying more than 16,000 are stranded in Yemen.
Some evacuations have taken place, with Russia on Sunday saying it had ferried more than 650 people of different nationalities out of Yemen by air and sea.
The IOM said Sunday it had also flown a first planeload of 141 passengers out of Sanaa.
Saudi Arabia has called on Iran to end its support for the rebels, accusing Tehran of assisting "criminal activities" in Yemen and providing weapons and aid to the Houthis.
Iran has denied arming the rebels and supreme leader Ayatollah Ali Khamenei has condemned the coalition's raids in Yemen as "criminal".
The conflict has raised fears of regional instability.
Al-Qaeda militants have taken advantage of the chaos to seize control of some areas, including the capital of one Yemeni province.
Before the escalation, Washington carried out a longstanding drone war against the local Al-Qaeda branch in Yemen.