The African Union said Friday it had launched an investigation after its Twitter account lashed out at US aid chief Samantha Power for urging dialogue to end Ethiopia's nine-month-old war.

Power, who visited Ethiopia this month, retweeted a post by US national security adviser Jake Sullivan that called on "all parties to urgently come to the negotiating table."

She then wrote that the "dire" humanitarian situation in Ethiopia "will get much worse unless all parties agree to dialogue & ending hostilities".

The account for the AU, which is headquartered in the Ethiopian capital Addis Ababa, responded swiftly -- and caustically.

"Good Lord, you mean just like the time you sat and talked to ISIS and Taliban?" its post said, before using a hashtag to refer to rebels from Ethiopia's war-hit Tigray region as terrorists -- which is also the Ethiopian government's position.

The post was deleted, and the AU said it was looking into the matter.

"An (AU) staff member shared personal views on this handle, contrary to AU staff rules. The now deleted tweet does not reflect the AU position, and is viewed as a serious breach of conduct. The incident is being investigated internally," it said.

An AU official confirmed to AFP that the author of the offending post is Ethiopian.

Northern Ethiopia has been wracked by conflict since November, when Prime Minister Abiy Ahmed sent troops into the northernmost region of Tigray to topple the regional ruling party, the Tigray People's Liberation Front (TPLF), saying the move came in response to attacks on army camps.

Abiy declared victory several weeks later when government forces took the regional capital Mekele, but the rebels mounted a shock comeback, recapturing the city and most of Tigray by late June.

Since then they have pressed east and south into the neighbouring Afar and Amhara regions, seizing a host of towns including Lalibela, a UNESCO World Heritage Site in Amhara that is home to mediaeval rock-hewn churches.

The Tigray war has proved sensitive for the AU.

Abiy rejected early appeals from high-level AU envoys for talks with Tigrayan leaders, sticking to his line that the conflict is a limited "law and order" operation.

A commission of inquiry on Tigray by the African Commission on Human and Peoples' Rights began work in June.