The Saudi-led coalition is believed to be behind the offensive, having stepped up operations on areas held by the Iranian-backed Houthi rebels.

One of the targets, where at least 70 people died, was a prison run by the Houthis near the northern city of Saada, according to Houthi health minister Taha al-Motawakel.

At least 100 others were injured, Save the Children said, while Doctors Without Borders said the figure was "around 200".

Aid workers continue to clear the rubble with more casualties expected to be discovered.

Save the Children said most of those killed were detained migrants held in the jail.

The children who died were said to have been playing on a nearby football field when missiles struck the port town of Hudaida on Friday morning.

The airstrikes also damaged a telecommunications centre in Hudaida, which resulted in a nationwide internet blackout.

The internet disruption started at around 1am local time and affected TeleYemen, the state-owned monopoly that controls internet access in the country and is now run by the Houthis, according to NetBlocks.

Over 12 hours later, the internet remained down.

The Norwegian Refugee Council said the strike was "a blatant attack on civilian infrastructure that will also impact our aid delivery".

More airstrikes hit near Sanaa, Yemen's capital, which has been held by the Houthis since late 2014.

Thousands of demonstrators gathered in the city to express their outrage.

"The initial casualties report from Saada is horrifying," said Gillian Moyes, Save the Children's country director in Yemen.

"Migrants seeking better lives for themselves and their families, Yemeni civilians injured by the dozens, is a picture we never hoped to wake up to in Yemen."

The Saudi-led coalition acknowledged carrying out "accurate airstrikes to destroy the capabilities of the militia" around Hudaida's port.

Earlier this week the Houthis claimed responsibility for an unprecedented assault on the United Arab Emirates, a member of the coalition, which killed at least three people and injured at least six others.

The Houthis said they fired four Quds cruise missiles at an oil refinery in the Musaffah district and airport in Abu Dhabi, a Zulfiqar ballistic missile at Dubai airport and several drones at those and other sites.

United Nations envoy for Yemen Hans Grundberg voiced grave concern over the military escalation and called on both sides to "exercise maximum restraint".