UNITED NATIONS: People have started to die of hunger in Ethiopia's conflict-hit northern Tigray region where the humanitarian situation has deteriorated and sexual violence is still being used as a weapon of war, the UN's aid chief told the Security Council Thursday.

Mark Lowcock, the undersecretary-general for humanitarian affairs, said there was no evidence of a promised withdrawal of Eritrean troops from the region -- and urged them to make good on their pledge, according to a text of his speech seen by AFP.

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 Defence 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."

Lowcock 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 Defense 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.

The Eritrean government has previously denied reports that Eritreans have committed abuses against civilians, including mass killings and rapes.