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The death toll from a that hit a prison run by climbed to at least 80 detainees, the rebels said on Saturday.

The was part of an intense air and ground offensive that marked an escalation in Yemen’s years-long civil war. The conflict pits the internationally recognised government, aided by the Saudi-led coalition, against the Iranian-backed rebels.

The Houthis’ media office reported the death tally, saying that rescuers were still searching for survivors and bodies in the rubble of the prison site in the northern province of Saada on the border with Saudi Arabia.

The Doctors Without Borders charity said around in the airstrike.

The air attack, one of the deadliest of the war, renewed criticism of the coalition from the United Nations and international aid and rights groups.

Saudi coalition spokesman Brig. Gen. Turki al-Malki alleged the Houthis hadn’t reported the site as needing protection from airstrikes to the UN��or the International Committee of the Red Cross. He claimed the Houthis’ failure to do so represented the militia's “usual deceptive approach” in the conflict.

The Houthis used the prison to hold detained migrants, mostly Africans, attempting to cross through the war-torn country into Saudi Arabia.

The escalation was the most intense since 2018 fighting for the Red Sea port of Hodeida and comes after a year of US��and U.N. efforts failed to bring the two sides to the negotiating table.

The conflict in the Arab world’s poorest country began in 2014, when the Houthis took the capital, Sanaa, and much of northern Yemen, forcing the government to flee to the south, then to exile in Saudi Arabia.

The Saudi-led coalition, backed at the time by the U.S., entered the war months later to try to restore the government to power.

The conflict has turned into the world’s worst humanitarian crisis, with international criticism of Saudi airstrikes that have killed hundreds of civilians and targeted the country’s infrastructure. The Houthis meanwhile have used child soldiers and indiscriminately laid land mines across the country.