Chad's President Idriss Deby has died while visiting troops on the frontline of a fight against northern rebels, an army spokesman 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.

Mr Déby "breathed his last defending the sovereign nation on the battlefield", an army general said in a statement read out on state TV.

He had gone to the front line at the weekend to visit troops battling rebels based across the border in Libya.

The military council will be led by the late president's son, Mahamat Idriss Déby Itno, 37, a four-star general.

His campaign said on Monday 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, according  

The cause of death was not yet clear by press time.

Army spokesman Azem Bermendao Agouna announced his death in a broadcast on state television, surrounded by a group of 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 in order 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."

President Deby has been considered key 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.

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