However, the Saudi-led coalition said its airstrikes overnight on Dhamar targeted "a Houthi location for hostile air defense missiles and drone storages", Saudi-owned AL Arabiya television reported on Twitter.
The Saudi-led coalition's latest air raid comes months after President Donald Trump vetoed a War Powers resolution that would have ended USA military cooperation with Saudi Arabia's years-long assault on Yemen, which has created the world's worst humanitarian crisis.
The conflict has claimed tens of thousands of lives, thrust millions to the brink of starvation and spawned the world's worst humanitarian crisis.
Earlier, ICRC said it has sent "urgent medical supplies" to the area.
A line of over a dozen white body bags were laid out in the rubble beside flattened buildings and crushed cars, while rescue workers dug through the debris.
He said the Yemeni Red Crescent Society was still trying to retrieve bodies and that around 50 injured people had been taken to hospital. "The scale of the casualties is staggering".
The coalition, whose rights record has been decried by global rights groups, said it had taken measures to protect civilians in Dhamar, claiming the assault complied with worldwide law.
"Researchers were unable to identify military targets in numerous strikes investigated in this cluster, calling into question the legitimacy of the attacks", it said.
"The explosions were strong and shook the city", a resident said.
"This is a traditional Huthi tactic and a violation of the laws of armed conflict".
At least 50 killed in Saudi-led raid on Houthi prison in Yemen
The coalition said it "took all precautionary measures to protect civilians". "They were targeting the jail, I really don't know the strike numbers", wounded detainee Nazem Saleh said while on a stretcher in a local hospital.
"The targeted prison housed over 170 prisoners of war, most of whom were supposed to be part of a local exchange deal", he was quoted as saying by the Houthi-run al-Masirah TV.
Shortly following the bombing, United Nations special envoy to Yemen Martin Griffiths issued a statement demanding an investigation and accountability for the attack.
"The human cost of this war is unbearable". "We need it to stop".
The attack was the deadliest so far this year by the coalition, according to the Yemen Data Project, a database tracking the war.
The attack in Dhamar comes weeks after a new front opened in Yemen's complex war, pitting 2 of Riyadh's allies against each other in a struggle for control of the south.
The southern forces last month seized control of the interim capital Aden in the south, in a see-sawing battle with government troops that has drawn warnings that the country could break apart entirely. Activists launched an online petition collecting signatures to "kick Emiratis out of Yemen" and members of the Yemeni government issued a statement demanding the president end the UAE's role in Yemen.
A joint Saudi-Emirati committee met with the combatants and reached a ceasefire later in the day, the officials said.
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