The United Nations aid chief has told the Security Council that sexual violence is being used as a weapon of war in Ethiopia's Tigray. The US ambassador to the UN said Washington was "horrified by the reports of rape and other unspeakably cruel sexual violence" in the region, adding that Eritrean forces must be removed from Ethiopia "immediately". Mark Lowcock, the UN undersecretary-general for humanitarian affairs, told the Security Council that there was no evidence of a promised withdrawal of Eritrean troops from the region. He said people were dying of hunger in the northern Tigray area, where the humanitarian situation has deteriorated and sexual violence is still being used as a weapon of war.Lowcock gave a sobering assessment of the situation on the ground in Tigray during the closed-door meeting, the first in more than a month, requested by the United States."Unfortunately, I must say that neither the UN nor any of the humanitarian agencies we work with have seen proof of Eritrean withdrawal," Lowcock said."Without a ceasefire, this already-grave humanitarian crisis is only going to get a lot worse," said Lowcock, who this week received the first report of four internally displaced people dying from hunger."I again reiterate the need for Eritrean Defense Forces to halt atrocities and withdraw. Announcing it is not the same as doing it."The US ambassador to the UN, Linda Thomas-Greenfield, said in a statement that Washington was "horrified by the reports of rape and other unspeakably cruel sexual violence that continue to surface," as she warned that Eritrea "must withdraw its forces from Ethiopia immediately."Sexual violence as a weapon of warThe Eritrean government has denied reports that Eritreans have committed abuses against civilians, including mass killings and rapes.But the UN's chief humanitarian said sexual violence was on the rise in Tigray."There is no doubt that sexual violence is being used in this conflict as a weapon of war, as a means to humiliate, terrorise and traumatise an entire population today and into the next generation," Lowcock said, adding that majority of rapes were being perpetrated by the military."Cases reported have involved Ethiopian National Defence Forces, Eritrean Defence Forces, Amhara Special Forces and other irregular armed groups or aligned militia."Fighting needs to stop," Lowcock said, calling for humanitarian assistance to "dramatically scale up".As many as nine in 10 of the region's population of six million are estimated by the government to need emergency food assistance, he saidEritrean Defence Forces directly accusedLowcock said there have been reports of Eritrean soldiers wearing Ethiopian uniforms, adding: "Regardless of uniform or insignia, humanitarian staff continue to report new atrocities which they say are being committed by Eritrean Defence Forces."But the Eritreans are not the sole actor," he said. "There are reports of civilians being attacked and driven from their homes in western Tigray by Amhara militias, and Amhara authorities are restricting access to those people."Ethiopian Prime Minister Abiy Ahmed sent troops into Tigray in November to detain and disarm leaders of the once dominant regional ruling party, the Tigray People's Liberation Front (TPLF).He said the move came in response to TPLF-orchestrated attacks on federal army camps.