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

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

His campaign organisers said on Monday he was joining troops battling what he called “terrorists” after rebels based across the northern frontier in Libya advanced hundreds of kilometres south toward the capital N’Djamena.

The cause of his death was not yet clear.

Army spokesman Azem Bermendao Agouna announced his death in a broadcast on state television while surrounded by military officers he referred to as the “national council of transition”.

“A call to dialogue and peace is launched to all Chadians in the country and abroad to continue to build Chad together,” he said.

“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 region.

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

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