Airstrike on Yemen prison Photograph:( Twitter )
Further south in the port city of Hodeida, the children died when airstrikes by the Saudi-led coalition hit a telecommunications facility as they played nearby, Save the Children said. Yemen also suffered a country-wide internet blackout.
At least 70 people were killed in an air strike on a prison and at least three children died in a separate bombardment as Yemen's long-running conflict suffered a dramatic escalation of violence on Friday.
The Houthi rebels released gruesome video footage showing bodies in the rubble and mangled corpses from the prison attack, which levelled buildings at the jail in their northern heartland of Saada.
Further south in the port city of Hodeida, the children died when air strikes by the Saudi-led coalition hit a telecommunications facility as they played nearby, Save the Children said. Yemen also suffered a country-wide internet blackout.
"The children were reportedly playing on a nearby football field when missiles struck," Save the Children said.
The UN Human Rights Office said the reports of intensifying coalition air strikes on populated areas were "deeply shocking".
Eight aid agencies operating in Yemen said in a joint statement 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."
They said the prison in Saada was used as a holding centre for migrants, who made up many of the casualties.
But the United Arab Emirates insisted that it and other coalition members remained committed to "proportionate" responses to Huthi attacks.
"The coalition undertakes to abide by international law and proportionate response in all its military operations," UAE ambassador to the United Nations Lana Nusseibeh said.
The coalition attacks come after the Huthis took the seven-year war into a new phase by claiming a drone and missile attack on Abu Dhabi that killed three people on Monday.
The United Arab Emirates, part of the Saudi-led coalition fighting the rebels, had threatened reprisals.
Aid workers said hospitals were overwhelmed in Saada after the prison attack, with one receiving 70 dead and 138 wounded, according to Doctors Without Borders.
Two other hospitals have received "many wounded" and as night fell, the rubble was still being searched, the aid agency said.
Ahmed Mahat, Doctors Without Borders' head of mission in Yemen, said: "There are many bodies still at the scene of the air strike, many missing people."
"It is impossible to know how many people have been killed. It seems to have been a horrific act of violence."
The UN Security Council, meeting Friday at the request of non-permanent member the United Arab Emirates, unanimously condemned what it called the Huthis' "heinous terrorist attacks in Abu Dhabi... as well as in other sites in Saudi Arabia".
The coalition has been fighting the rebels since 2015, in an intractable conflict that has displaced millions of Yemenis and left them on the brink of famine.
The coalition claimed the attack in Hodeida, a lifeline port for the shattered country, but did not say it had carried out any strikes on Saada.
Saudi Arabia's state news agency said the coalition carried out "precision air strikes... to destroy the capabilities of the Huthi militia in Hodeida".
"Collapse of internet"
Global internet watchdog NetBlocks reported a "nation-scale collapse of internet connectivity". AFP correspondents in Hodeida and Sanaa confirmed the outage. Save the Children said it would hamper its operating capacity.
Yemen's civil war began in 2014 when the Huthis descended from their base in Saada to overrun the capital Sanaa, prompting Saudi-led forces to intervene to prop up the government the following year.
Tensions have soared in recent weeks after the UAE-backed Giants Brigade drove the rebels out of Shabwa province, undermining their months-long campaign to take the key city of Marib further north.
On January 3, the Huthis hijacked a United Arab Emirates-flagged ship in the Red Sea, prompting a warning from the coalition that it would target rebel-held ports.
And on Monday, they claimed a long-range attack that struck oil facilities and the airport in the UAE capital Abu Dhabi, killing two Indians and a Pakistani, and wounding six other people.
The attack -- the first deadly assault acknowledged by the UAE inside its borders and claimed by the Huthis -- opened up a new front in Yemen's war and sent regional tensions soaring.
In retaliation, the coalition carried out airstrikes against rebel-held Sanaa that killed 14 people.
Yemen's civil war has been a catastrophe for millions of its citizens who have fled their homes, with many close to famine in what the UN calls the world's worst humanitarian crisis.
The UN has estimated the war killed 377,000 people by the end of 2021, both directly and indirectly through hunger and disease.