Ethiopia's Prime Minister Abiy Ahmed replaced the heads of two branches of the security services as he presses ahead with wide-ranging political and economic reforms.

Sworn in as premier of the Horn of Africa country in April, Abiy has pledged to push through reforms in response to demands from protesters during waves of unrest that began in 2015, when security forces killed hundreds of people.

He named Seare Mekonnen as head of the armed forces in place of Samora Yunis, Abiy's Chief of Staff Fitsum Arega said in a Twitter post late on Thursday.

Samora had served in Ethiopia's military for four decades.

The premier also named air force head Adem Mohamed to head the NISS security service, replacing Getachew Assefa, the Ethiopian News Agency said. It quoted a statement from the prime minister's office.

The reasons for the changes were not disclosed, but both Samora and Getachew are senior members of the TPLF, the party that has dominated Ethiopian politics since 1991.

"This certainly indicates a weakening of the TPLF," said a university lecturer in the capital Addis Ababa who declined to give his name.

The TPLF is dominated by ethnic Tigrayans, while Abiy is an ethnic Oromos.