An excavator and a bulldozer shifted piles of concrete from buildings that collapsed in Sunday's attack on the rebel-held western city of Dhamar, in a video filmed by an AFP collaborator.

The strike hit a former college used as a detention centre by Huthi rebels.

The footage shows a bloody corpse being uncovered from among the splayed slabs and reinforced iron, as Red Crescent workers grabbed a white body bag.

Four filled body bags were placed side by side on the earth.

"The work continues -- the local teams, along with others coming from Sanaa, have extracted a number of bodies from the debris," said Hassan al-Ansi, a rescue worker.

He did not give a precise death toll or indicate how many bodies have been found during three days of combing through the rubble.

On Sunday, the International Committee of the Red Cross head in Yemen, Franz Rauchenstein, estimated more than 100 people had been killed in the strike.

An official at the health ministry run by the rebels, Dr Youssef al-Hadhiri, told AFP on Wednesday the death toll had reached 123, with 50 others injured.

The military coalition led by Saudi Arabia said Monday it "had not been informed" about the presence of prisoners at the targeted site.

The Saudi coalition began its intervention in 2015 in support of Yemen's internationally-recognised government against the Huthis, who still control swathes of western and northern Yemen.

"The Huthis bear full responsibility for making this a location for Yemeni citizens who have been forcibly disappeared," coalition spokesman Turki al-Maliki said in a press conference.

In Dhamar hospital, a wall has been filled with photos of more than 60 of the dead, as people try to identify loved ones.

"I came here to look for a martyr who had been detained for four months," said Saleh Ahmed, speaking of one of his relatives.

The wounded roamed corridors of the hospital, looking lost, as others watched a speech by rebel leader Abdelmalek al-Huthi on television from their beds.

Since 2014, Yemen's conflict has killed tens of thousands of people, mostly civilians.

It has plunged the country -- the poorest in the Arab world -- into the globe's worst humanitarian crisis, according to the UN.

A UN war crimes investigators said Tuesday they had "identified, where possible, individuals who may be responsible for international crimes" in Yemen and had provided a confidential list to UN rights chief Michelle Bachelet.