A US envoy will head to Ethiopia to urge an end to fighting as a war centered on the Tigray region widened, worsening fears of a humanitarian disaster, officials said late on Thursday.

Jeffrey Feltman, a veteran diplomat serving as US special envoy for the Horn of Africa, will visit Ethiopia on an August 15-24 trip that will also take him to Djibouti and the United Arab Emirates, the State Department said.

President Joe Biden asked Feltman to return to Ethiopia at a "critical moment," national security advisor Jake Sullivan said.

"Months of war have brought immense suffering and division to a great nation that won't be healed through more fighting," Sullivan wrote on Twitter.

"We call on all parties to urgently come to the negotiating table," he said.

Feltman's mission follows a trip earlier this month to Addis Ababa by US aid chief Samantha Power, who pushed all sides to end hostilities and warned of an impending "humanitarian catastrophe" if more aid cannot enter.

Prime Minister Abiy Ahmed in November launched an offensive against Tigray's then ruling party in response to its attacks on federal army camps.

Nine months later, the Tigray People's Liberation Front has made advances into the neighboring Afar and Amhara regions and it recently unveiled an alliance with rebels from Oromia, Ethiopia's largest region.

Aid workers have been struggling to enter Tigray with the United Nations estimating that 400,000 people there risk famine.