UN Secretary General Ban Ki-moon called for international action to end the Saudi-led air campaign on Huthi rebels as intense bombing hit Yemen again Friday and al Qaeda seized more ground in the chaos.
Columns of smoke rose over an arms depot targeted by warplanes east of the capital Sanaa, witnesses said.
The facility belonged to the elite Republican Guard, which remains loyal to former president Ali Abdullah Saleh.
Renegade troops loyal to Saleh are allied with the Huthi rebels, whose sweeping advance forced President Abedrabbo Mansour Hadi to flee to Riyadh last month.
Following heavy overnight air strikes in the north, coalition aircraft also hit the presidential palace in the southern city of Taez, the witnesses said.
Coalition spokesman Brigadier General Ahmed al-Assiri said that "from this afternoon we have started operations in Taez".
Speaking in Riyadh, he added that there had been 100 sorties in Yemen on Thursday, indicating there is no end in sight to the operation.
"This works needs patience, persistence and precision. We are not in a hurry... We have the time and we have the capabilities."
Air strikes on the southern port city of Aden killed a rebel, while at least 76 other people died in bombing and fighting around Aden and Taez, officials said.
The United Nations says hundreds of people have died and thousands of families fled their homes in the war, which has also killed six Saudi security personnel in border skirmishes.
Ban called for an immediate ceasefire, saying the country was "in flames" and all sides must return to political negotiations.
His remarks followed the resignation of envoy Jamal Benomar, saying he wanted to move on to a new assignment, but diplomats confirmed that he had lost the support of Yemen's exiled president and Gulf countries.
Saudi Arabia and its neighbours accuse Benomar of being duped by the Shiite Huthis who took part in peace negotiations as they pushed their offensive.
The Moroccan diplomat had been instrumental in negotiating a deal that eased Saleh from office in February 2012 after a year of protests against his three-decade rule.
Qaeda overruns Mukalla
On Friday, al Qaeda overran a key army camp in the Hadramawt provincial capital Mukalla, seizing heavy weapons and consolidating its grip on the city, an official said.
Residents confirmed that the camp, which had remained loyal to Hadi, was taken "without resistance".
Despite the collapse of Hadi's government in Yemen, Washington has vowed to carry on its campaign against al Qaeda in the Arabian Peninsula.
Two suspected AQAP militants were killed in an apparent US drone strike in Shabwa province in the south overnight, a tribal source said.
The World Health Organization, in a new toll, says 767 people have died in Yemen's war since March 19 and more than 2,900 were wounded. The majority have been civilians.
The United Nations launched an urgent appeal for $274 million (253 million euros) to provide emergency aid for what was already the region's poorest country.
"Ordinary families are struggling to access health care, water, food and fuel -- basic requirements for their survival," UN Humanitarian Coordinator Johannes Van Der Klaauw said.
The White House said President Barack Obama will host leaders of the six-nation Gulf Cooperation Council on May 13-14 to discuss Yemen and other regional issues.
Meanwhile, a Norwegian journalist, detained in Huthi-controlled Sanaa in late March, has been freed and is on his way home, Oslo's foreign ministry said.