WASHINGTON: The United States on Friday urged Americans in Ethiopia to leave without delay, as fears mount of Tigrayan rebel fighters advancing on the capital Addis Ababa.

"The security environment in Ethiopia is very fluid. We advise US citizens who are in Ethiopia to leave the country as soon as possible," the US Embassy in Ethiopia said in a statement.

It went further than a warning earlier this week by the State Department for Americans to avoid traveling to Ethiopia and consider leaving if they are already there.

Nine rebel groups battling Ethiopia's government said Friday they will "collaborate and join forces" to defeat the government of Prime Minister Abiy Ahmed.

The alliance, due to be signed in Washington, includes the Oromo Liberation Army and the Tigray People's Liberation Front, which has been locked in a year-long war against the government.