On Friday, Mr. Mahat said the latest airstrikes had also hit Sana and its airport, and that the aid group had received numerous reports of overnight airstrikes elsewhere across northern Yemen.But none appeared to have been as deadly as the attack on the prison in Saada. No other information about the victims was immediately available, but Save the Children said early reports indicated that most were African migrants, who attempt to cross through Yemen on their way to seek work in Saudi Arabia and the other Gulf countries.The Houthis first swept to power in the aftermath of the 2011 Arab Spring uprising against Yemen’s authoritarian dictator, Ali Abdullah Saleh, whose successor, his deputy, struggled to contend with Yemen’s corruption, unemployment and a separatist movement.After they overran the capital in 2014 and 2015, forcing the Saudi-backed government to flee, the Saudi-led coalition began targeting them, fearing that their Iranian sponsors would gain a foothold in Saudi Arabia’s backyard.Now divided between Houthi control in the north and Saudi-backed government control in the south, Yemen has become the site of what aid groups say is one of the world’s worst humanitarian disasters, with millions of people living in famine-like conditions, an economy in collapse and basic services, including many hospitals, in tatters.Within a month of taking office, Mr. Biden had promised to push for ending the war in Yemen, partly by cutting off arms sales to Saudi Arabia. Yet as the Houthis gained ground last year, the Biden administration announced in November that it would sell $650 million of air-to-air missiles, which it classified as defensive weaponry, to the kingdom.The Saudi-led bombings Friday came on the same day that the United Nations Security Council, meeting at the request of the U.A.E., unanimously condemned what the council called the “heinous terrorist attacks in Abu Dhabi” earlier in the week, as well as on sites in Saudi Arabia.