A senior Houthi leader said efforts to convene U.N.-backed peace talks for Yemen were failing and blamed the United States and Saudi Arabia for continued aggression. The U.N. envoy Ould Cheikh Ahmed said last week he was hoping to begin preparatory talks with the government and Houthis separately and expected formal talks between the sides "in the coming weeks." "All understandings for a political solution leading to the cessation of aggression have failed," Houthi spokesman Saleh al-Samad wrote on Facebook. An Arab coalition has been carrying out airstrikes on Houthi positions for seven months, seeking to restore the Yemeni government forced into exile last year after the Iran-allied rebels took over the capital of Sanaa. The Houthis accuse the coalition of acting to further U.S. ambitions to dominate the region. Saudi Foreign Minister Adel al-Jubeir told a news conference in Riyadh Thursday that the bombing campaign may soon end, noting the acceptance of U.N. Security Council Resolution 2216 by the Houthis and affiliated militias. The U.N envoy wants the talks to focus on main elements of the resolution – withdrawing militias from key cities, releasing prisoners, returning heavy weapons seized from the army, improving the humanitarian situation and resuming dialogue. Houthi leaders endorsed the plan in early October and agreed to join the U.N.-backed talks. However Samad's statement called for the Houthis to intensify their bid for power. "We must redouble our efforts and exert ourselves to the utmost, ensuring the sacrifices made by our people over the past months do not go to waste," he wrote. Previous U.N.-sponsored peace talks faltered in June after President Abed Rabbou Mansour Hadi's government demanded the Houthis pull out of cities captured since last September as a precondition for a cease-fire. Yemeni security officials say the country's Shiite rebels have released 20 detainees in the western province of Ibb after pressure from local non-government groups. The officials said Friday the 20 were taken several weeks ago ahead of a planned march to the city of Taiz, to protest its siege by the rebels. The officials, who are neutral in the conflict that has splintered Yemen, spoke on condition of anonymity because they are not authorized to talk to reporters. For five months, the Houthis have been blocking food and medicine deliveries to Taiz, Yemen's third-largest city. The U.N. World Food Program Friday appealed for safe access to Taiz, saying that fighting between warring factions had blocked food supplies and left thousands of people in extreme hunger. The last U.N. food aid to reach Taiz was more than five weeks ago when food was distributed to nearly 240,000 people, it said. "We plead for safe and immediate access to the city of Taiz to prevent a humanitarian tragedy as supplies dwindle, threatening the lives of thousands – including women, children and the elderly," WFP regional director Muhannad Hadi said in a statement. "These people have already suffered extreme hunger, and if this situation continues the damage from hunger will be irreversible." Ten of Yemen's 22 governorates were assessed as being in an emergency food situation in June, one step below famine on a five-point scale. The assessment has not been updated since then, partly because experts have not managed to get sufficient access to survey the situation. About a third of the country's population, or 7.6 million people urgently require food aid, the WFP said. Doctors Without Borders (MSF) demanded answers Thursday from the coalition over the bombing of one of the medical charity's hospitals in northern Yemen, rejecting Saudi Arabia's denial of responsibility. The hospital in the northern city of Saada was hit late Monday drawing condemnation from United Nations chief Ban Ki-moon, who also fingered the coalition forces as responsible. The Saudi mission to the United Nations said in a statement that "the Arab coalition aircrafts did not attack the hospital" and were not in Saada at the time. "There have been two contradictory statements by the Saudi ambassador to the U.N.," said Mego Terzian, president of MSF France. "First he announced that the strike was probably a mistake and that MSF had given incorrect GPS coordinates. And on Oct. 29, the same ambassador denied any strikes in Saada division," he said. "At the moment we favor direct negotiations with those responsible, therefore the coalition, to clarify what happened," Terzian said. "If the negotiations do not produce convincing results, then we will probably take radical steps and withdraw from some areas, or even in the country as a whole," he warned.