An army spokesman announced on Tuesday that Chad’s President Idriss Deby has died while visiting troops on the frontline of a fight against northern rebels, the day after Deby was declared the winner of a presidential election.

68-year-old Deby came to power in a rebellion in 1990 and was one of Africa’s longest-ruling leaders.

On Monday, his campaign said he was joining troops battling what he called terrorists after rebels based across the northern frontier in Libya advanced hundreds of km (miles) south toward the capital N’Djamena.

The cause of death is yet to be clarified.

Army spokesman Azem Bermendao Agouna announced the president’s death in a broadcast on state television, surrounded by a group of military officers he referred to as the National Council of Transition.

He said “a call to dialogue and peace is launched to all Chadians in the country and abroad in order to continue to build Chad together.”

“The National Council of Transition reassures the Chadian people that all measures have been taken to guarantee peace, security and the republican order.”

Western countries have seen Deby as an ally in the fight against Islamist extremist groups, including Boko Haram in the Lake Chad Basin and groups linked to al Qaeda and Islamic State in the Sahel.

Deby was also dealing with growing public resentment over his management of Chad’s oil wealth and crackdowns on opponents.

His election victory had given him a 6th term in office but the April 11 vote was boycotted by opposition leaders.