Yemeni rebels demanded a Saudi-led coalition completely end its attacks as a condition for UN-sponsored talks, as the alliance launched new air strikes Wednesday a day after declaring its month-long campaign over.
The fresh raids broke a brief lull after the coalition announced Tuesday night that the first phase of its "successful" bombing campaign had finished and that it was now focusing on political efforts.
However, the coalition had warned it stood ready to counter any advance by the Iran-backed Huthi rebels and their allies.
And it duly responded with more firepower when the Huthis took advantage of the cessation and overran the headquarters of the 35th Armoured Brigade loyal to President Abedrabbo Mansour Hadi in third city Taez.
Clashes between the rebels and loyalists left dozens dead and wounded in a string of battleground towns, including second city Aden, an army officer said.
In their first statement since the coalition announcement, the Shiite rebels demanded a complete halt to attacks as a condition for UN-sponsored talks.
"We demand, after a complete end to the aggression against Yemen and the lifting of the blockade, to resume political dialogue... under the sponsorship of the United Nations," said spokesman Mohammed Abdulsalam.
The Saudi ambassador to the US reiterated that "if the Huthis or their allies make any aggressive moves there will be a response".
The UN had sponsored a Gulf-brokered peace deal that eased former president Ali Abdullah Saleh out of office in 2012, ending a year of bloody protests against his three-decade rule.
- Situation 'catastrophic' -
UN Secretary General Ban Ki-moon appealed for an end to the fighting and offered the "diplomatic facilities through which we can resolve this issue through dialogue".
Ban said he had proposed a successor to UN envoy Jamal Benomar, who resigned last week after reportedly losing support among Gulf countries.
The World Health Organization says at least 944 people have been killed in Yemen since March 19.
Riyadh said the strikes, which it launched on March 26 as the rebels closed in on Hadi's last refuge in Aden, had succeeded in eliminating the threat posed to Saudi Arabia and its neighbours by the rebels' air and missile capabilities.
But rebels remain in control of Sanaa and swathes of the country while Hadi is in exile in Riyadh, where he fled when the raids began.
The coalition said its operations would now enter a political phase with the focus on resuming talks, aid deliveries and "fighting terrorism".
The Red Cross warned of a "catastrophic" humanitarian situation, with fuel supplies reaching "zero levels" and an acute shortage of food leading to soaring prices.
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Al-Qaeda's Yemeni branch, regarded by Washington as its most dangerous, has taken advantage of the conflict to consolidate its grip on Hadramawt province in the southeast.
Seven suspected Al-Qaeda militants were killed in an apparent US drone strike on the provincial capital Mukalla, which the jihadists overran this month, sources said.
US Defence Secretary Ashton Carter has acknowledged Al-Qaeda is gaining ground but vowed America's longstanding drone war will go on.
- Obama 'proxy war' warning -
Washington, which has given intelligence and logistical support to the coalition, welcomed the end of the air campaign against the rebels.
US President Barack Obama called on Tehran to help find a political solution.
"What we need to do is bring all the parties together and find a political arrangement. It is not solved by having another proxy war in Yemen," he said.
"We've indicated to the Iranians that they need to be part of the solution, and not part of the problem."
UN-brokered talks between the warring parties broke down in February when Hadi fled to Aden after the Huthis seized Sanaa.
Former president Saleh, who has provided key support to the rebels, said he hoped the halt to the air war would lead to a return to dialogue.
"We hope that everyone will cooperate to return to dialogue, to find solutions other than placing losing bets that are wrong and costly," he said.
Army units that remained loyal to Saleh after his ouster have provided crucial support to the rebels.
In an apparent goodwill gesture, the rebels freed three top commanders -- including the defence minister and a brother of Hadi -- whom it had captured in the past month, mediators said.
Iran offered its help in bringing the sides back to the negotiating table.
"Positive developments in Yemen should be followed by urgent humanitarian assistance, intra-Yemeni dialogue & broad-based govt. Ready to help," Foreign Minister Mohammad Javad Zarif tweeted.
In a televised speech from Riyadh, Hadi thanked the coalition and refused to give up hope of returning from exile.
"We will soon return to our homeland, to Aden and Sanaa," he said.