In Riyadh, Yemen’s Prime Minister Khaled Bahah was sworn in as vice president at the country’s embassy in front of exiled President Abed Rabbou Mansour Hadi.

Hadi, meanwhile, in a New York Times opinion editorial accused Iran of fueling a “campaign of horror and destruction” by the Shiite Houthi rebels and being “obsessed with regional domination.”

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.

Farther north, fierce clashes continued in Daleh, where 40 rebels and three pro-president fighters have been killed since Sunday, a local official and southern fighters said.

Three children were killed Monday when a shell hit their house in south Daleh, southern activist Saleh Binlaghbar said.

Residents said warplanes from a Saudi-led coalition pounded the rebel-held presidential complex and other positions in Aden, Hadi’s last refuge before he fled to neighboring Saudi Arabia as the air war began on March 26. The Houthis, who have joined with forces loyal to ex-President Ali Abdullah Saleh, advanced on Aden last month after seizing Sanaa last year.

Saudi Arabia has accused Iran, the main Shiite power, of backing the rebel offensive and has rallied support for the coalition from regional allies.

Western powers have also backed Hadi as Yemen’s legitimate ruler, while the United Nations has called for talks to resume.

Human Rights Watch urged the coalition to take “necessary steps to minimize harm to civilians,” urging the bloc and the United States to investigate “alleged laws-of-war violations,” including the bombing of a displaced persons camp last month.

Humanitarian groups have struggled to deliver aid and said Monday the situation in Aden was deteriorating rapidly.

“Shops are closed. We have a problem of food,” said Marie-Elisabeth Ingres, Yemen representative of Doctors without Borders (MSF).

Metaz al-Maisuri, an activist in Aden, said basic services had ceased and there had been a “mass exodus” from the city.

Adwaa Mubarak, a 48-year-old Aden resident, said: “We are unable to leave our houses to buy what we need because of the Houthi snipers.”

U.N. Secretary-General Ban Ki-moon warned Sunday of a huge humanitarian crisis as “civilian casualties are mounting and public infrastructure is being destroyed.”

Aid workers said Sanaa is also suffering, as airstrikes hit rebel positions there and supplies dwindle.

The International Committee of the Red Cross flew more than 35 tons of medical aid and equipment into Sanaa Saturday, after Friday’s first deliveries.

HRW’s U.N. and crisis advocacy director, Philippe Bolopion, said “the alarming civilian deaths and humanitarian crisis in Yemen should spur the Saudi-led coalition and the Houthis to get serious about protecting civilians.”

HRW also noted that the rebels have “unlawfully” deployed forces in densely populated areas.

Thousands of foreigners have been trapped by the fighting, with the International Organization for Migration saying more than 16,000 are stranded.

Some evacuations have taken place, with Russia Sunday saying it had evacuated more than 650 people, and the IOM said it had flown a first planeload of 141 passengers out of Sanaa.

Riyadh has called on Iran to end its support for the rebels, accusing Tehran of assisting “criminal activities” in Yemen and giving the Houthis weapons and aid.

Iran has denied the allegation, and Supreme Leader Ayatollah Ali Khamenei has condemned the coalition’s raids on Yemen as “criminal.”

Hadi in his opinion piece accused the rebels of being Iran’s “puppets.”

“Having a hostile government in a nation bordering the Bab al-Mandab strait – the highly trafficked shipping lane leading to the Suez Canal – is in no nation’s interest,” he wrote.

“If the Houthis are not stopped, they are destined to become the next Hezbollah, deployed by Iran to threaten the people in the region and beyond.

“The oil shipments through the Red Sea that much of the world depends on will be in jeopardy, and Al-Qaeda and other radical groups will be allowed to flourish.”

Also Monday, Yemeni tribesmen seized control of army bases assigned with protecting Balhaf port, location of the country’s only gas export terminal, tribal and military sources said.

Soldiers from two army brigades based near the port in Shabwa province have abandoned their positions, and tribesmen who had besieged the bases advanced and seized heavy weaponry, the sources said.

The bases are just a few kilometers from the gas plant, in which France’s Total has an almost 40 percent interest.