Saudi-led coalition warplanes resumed strikes on rebel positions in southern Yemen on Monday after a five-day ceasefire expired, jeopardising efforts to deliver desperately needed aid.
The humanitarian truce ended at 2000 GMT Sunday despite calls from a UN envoy and relief agencies for an extension.
An hour later, air raids hit the rebel-held presidential palace in the main southern city Aden as well as a base of the pro-rebel special forces there, military officials and witnesses said.
The coalition also targeted rebel positions in Aden's neighbourhoods of Tawahi and Khor Maksar, said the sources, without providing details on casualties.
The Saudi-led regional coalition has waged an air war on the Iran-backed Shia Houthis rebels and their allies since late March in an effort to restore the authority of exiled President Abedrabbo Mansour Hadi.
The new strikes came despite calls by UN envoy to Yemen Ismail Ould Cheikh Ahmed to prolong the ceasefire.
"This humanitarian truce should turn into a permanent ceasefire," he said Sunday in the Saudi capital.
A Western diplomat in Riyadh said Saudi Arabia has accused the Houthis of using the ceasefire to position long-range artillery, rockets and other weapons close to its border.
"They say this is a direct violation of the ceasefire agreement," the diplomat said.
The Saudi-owned Al-Arabiya news channel said Saudi forces fired artillery at Houthi positions over the border in Yemen. It said the rebels hit a crossing point in Najran province.
Yemeni political parties began talks Sunday in the Saudi capital but the Houthis are boycotting the three-day meeting, insisting the talks be held in Yemen.
However, Yemen's Vice-President Khaled Bahah told AFP that he expected the Houthis would attend future talks.
Several representatives of the General People's Congress party of ousted president Ali Abdullah Saleh, who is accused of backing the rebels, did take part.
Hadi, who has taken refuge in Riyadh, on Sunday repeated accusations that the rebels had staged a "coup".
"We are trying to regain our nation" from militias backed by "external" forces, he said in a reference to Iran, which has denied arming the rebels.
Sporadic clashes continued Monday in Aden, the scene of fierce fighting since rebels advanced on the southern port in late March where Hadi was holed up.
According to Aden health chief Al-Khader Laswar, 517 civilians and pro-Hadi fighters have been killed in the city in the past 50 days.
The toll includes 76 women and children, he said.
Quoted by the government news agency Sabanew.net, Laswar said he could not provide a toll for the rebels.
He added that 3,461 people were wounded, and said most Aden hospitals were now out of service as "most" medics have fled.
Pro-Hadi fighters also killed 12 rebels in an attack Monday on a base in Daleh, a military official said.
The United Nations has expressed deep concern about the civilian death toll from the Saudi-led bombing as well as the humanitarian impact of an air and sea blockade imposed by the coalition.
It says more than 1,600 people have died in the conflict since late March.
Some aid has trickled in since the pause in fighting, but residents of areas where clashes persist complain that they lack the most basic supplies.
Although the truce had brought calm to the capital and some other areas, people remained deprived of basic needs, including water, electricity and petrol.
Hundreds of cars have lined up at the few petrol stations operating, according to an AFP correspondent.
The ceasefire allowed supplies of petrol and food to be delivered to Yemen but anti-Houthi groups accuse the rebels of confiscating the aid.
"The truce has only served the aims of the militia, which has increased its readiness and stocked fuel through aid that arrived in their areas of control," said a statement of anti-rebel forces in the central city of Taez.