The Saudi-led coalition on Saturday denied carrying out an air strike on a prison in Yemen's rebel-held north that aid groups said killed at least 70 people, including migrants, women and children.

Claims the coalition ordered the raid, which reduced buildings to rubble and left rescuers scrabbling for survivors with their bare hands, were "groundless", the alliance said.

An image grab from video made available by Yemen's Huthis shows what the rebels say is the prison destroyed by the attack on Saada An image grab from video made available by Yemen's Huthis shows what the rebels say is the prison destroyed by the attack on Saada Photo: Ansarullah media center via AFP

The attack, which coincided with a coalition strike on the Yemeni port of Hodeida that killed three children and knocked out the impoverished country's internet, was condemned by UN chief Antonio Guterres.

But "these claims adopted by the militia are baseless and unfounded", said coalition spokesperson Turki al-Malki, referring to the Iran-backed rebels.

This week has witnessed a dramatic upswing in the conflict that has already killed tens of thousands of people and displaced millions, creating what the UN calls the world's worst humanitarian crisis.

Map of Yemen locating Hodeida and Saada Map of Yemen locating Hodeida and Saada Photo: AFP / Sophie RAMIS

The rebels took the capital Sanaa in 2014, prompting the Saudi-led intervention -- supported by the US, France and Britain -- in March 2015. It was intended to last just a few weeks.

The latest violence came after the Huthi rebels on Monday claimed their first deadly attack on Abu Dhabi, capital of coalition partner the United Arab Emirates, taking the conflict into a new phase.

The drone and missile attack, which killed three people, was the first deadly assault the UAE has acknowledged inside its borders, and prompted threats of reprisals.

Supporters of Yemen's Huthi rebel movement marched through the capital Sanaa to denounce attacks by the Saudi-led coalition Supporters of Yemen's Huthi rebel movement marched through the capital Sanaa to denounce attacks by the Saudi-led coalition Photo: AFP / MOHAMMED HUWAIS

Supporters of Yemen's Huthisdemonstrate in the capital Sanaa to denounce a reported air strike by the Saudi-led coalition on a prison Supporters of Yemen's Huthisdemonstrate in the capital Sanaa to denounce a reported air strike by the Saudi-led coalition on a prison Photo: AFP / MOHAMMED HUWAIS

The internet blackout, which went into its second day on Saturday according to web monitor NetBlocks, complicated rescue work and media reporting as information slowed to a trickle.

"The nation-scale internet disruption in Yemen is ongoing with no indication of recovery," NetBlocks said.

Unverified footage released by the Huthis revealed gruesome scenes at the bombed-out prison facility, as rescue workers scrambled to dig out bodies and mangled corpses were placed in piles.

Yemeni pro-government fighters, with one pictured here in this December 7, 2021 photograph, have been battling Huthi forces for over seven years Yemeni pro-government fighters, with one pictured here in this December 7, 2021 photograph, have been battling Huthi forces for over seven years Photo: AFP

Eight aid agencies operating in Yemen said in a joint statement that the prison in Saada, the rebels' home base, was used as a holding centre for migrants, who made up many of the casualties.

They said they were "horrified by the news that more than 70 people, including migrants, women and children, have been killed... in a blatant disregard for civilian lives".

Hospitals were overwhelmed as hundreds of casualties flooded in, aid workers said.

"It is impossible to know how many people have been killed. It seems to have been a horrific act of violence," said Ahmed Mahat, Doctors Without Borders' head of mission in Yemen.

Meeting on Friday, the UN Security Council unanimously condemned the "heinous terrorist attacks" on Abu Dhabi, but the council's Norwegian presidency also denounced the strikes on Yemen.

In a later statement, the UN chief reminded "all parties that attacks directed against civilians and civilian infrastructure are prohibited by international humanitarian law".

US Secretary of State Antony Blinken called for "all parties to the conflict to de-escalate" and "abide by their obligations under international humanitarian law".

Iran on Saturday also condemned recent air strikes on Huthi-held areas, warning they have "made the path to achieve a just peace in the country even more difficult," foreign ministry spokesman Saeed Khatibzadeh said.

Saudi Arabia accuses regional rival Iran of providing military support to the Huthis, especially missiles and rockets, claims that Tehran denies.

Khatibzadeh said there was a lack of "serious determination to advance the political settlement of the Yemeni crisis", warning it would lead to the "destruction of the country and instability in the region".

The Huthis have warned foreign companies to leave the "unsafe" UAE, a veiled threat of revenge attacks after Friday's strikes.

"We advise the foreign companies in Emirates to leave because they invest in an unsafe country and the rulers of this country continue in their aggression against Yemen," warned Huthi military spokesperson Yahya Saree.