Chad's President Idriss Deby, who ruled his country for more than 30 years and was an important Western ally in the fight against Islamist militants in Africa, has been killed in a battle against rebels in the north.

His son, Mahamat Idriss Deby Itmo, was named interim president by a transitional council of military officers, army spokesman Azem Bermendao Agouna said on state television.