Under a November truce that ended a recent war between Israel and Iran-backed Hezbollah, the army has been deploying in south Lebanon and dismantling the militant group's infrastructure there.

"Five soldiers were killed in an explosion... inside a Hezbollah military facility," the source said, requesting anonymity as they were not authorised to brief the media.

The blast erupted as the troops were "removing munitions and unexploded ordnance left over from the recent war" between Israel and Hezbollah, the source said.

The army did not immediately issue a statement.

But President Joseph Aoun said he spoke to army commander Rodolphe Haykal about a "painful incident" near Majdal Zoun and Wadi Zibqin in Tyre district that led to an unspecified number of casualties among troops.

The presidency statement said the blast was due to a munitions explosion as an engineering unit "was working to remove and disable" the ordnance.

Prime Minister Nawaf Salam paid tribute on X to the troops who were killed "while performing their national duty".

The announcement came after Andrea Tenenti, spokesperson for UN peacekeepers in Lebanon, said Thursday that troops had "discovered a vast network of fortified tunnels" in the same area.

UN spokesman Farhan Haq had told reporters that peacekeepers and Lebanese troops found "three bunkers, artillery, rocket launchers, hundreds of explosive shells and rockets, anti-tank mines and about 250 ready-to-use improvised explosive devices".