File Photo: Soldiers of Tigray Defence Force (TDF) walk in lines towards another field in Mekele, the capital of Tigray region, Ethiopia, on June 30, 2021. AFP
The warning came after the Tigray People's Liberation Front (TPLF) rebel group announced late Wednesday it had reached the town of Kemissie in Ethiopia's Amhara region, some 325 kilometres (200 miles) northeast of Addis Ababa.
Spokesman Getachew Reda said the TPLF was working in the area alongside the Oromo Liberation Army rebel group, which this week predicted Addis Ababa could fall in a matter of weeks.
"We can only assume that any march towards Addis would spread increased displacement, increased need and increased suffering for the Ethiopian people," said the senior official from Washington's humanitarian arm USAID.
"It would certainly increase the need for humanitarian assistance while also complicating the ability to provide that assistance," the official said.
The US has called on all parties to cease hostilities, and its top envoy for the Horn of Africa, Jeffrey Feltman, was due to arrive in Addis Ababa Thursday for two days of talks promoting peace and dialogue.
The US embassy announced Thursday it was authorising the voluntary departure of most staff and their families, although no one is obligated to leave at this stage.
"Travel to Ethiopia is unsafe at this time due to the ongoing armed conflict. Incidents of civil unrest and ethnic violence may occur without warning," said an advisory posted on the embassy's website.
"Further escalation is likely, and may cause supply chain shortages, communications blackouts and travel disruptions."
Washington has been among the most vocal critics of Ethiopia's conduct of the year-long war in the country's north.