Air strike on Yemen prison leaves 70 dead

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 Houthi 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 Houthis 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.