The Lebanese army launched an offensive on Saturday against an Islamic State enclave on the northeast border with Syria as Hezbollah simultaneously announced an assault on the militants from the Syrian side of the border.
The Lebanese army was targeting Islamic State positions near the town of Ras Baalbek with rockets, artillery and helicopters, a Lebanese security source said. The area is the last part of the Lebanese-Syrian frontier under insurgent control.
The operation by Hezbollah and the Syrian army was aimed at Islamic State militants in the western Qalamoun region of Syria, Hezbollah said, an area across the frontier from Ras Baalbek.
A Hezbollah statement said the group was meeting its pledge to "remove the terrorist threat at the borders of the nation" and was fighting "side by side" with the Syrian army. It made no mention of the Lebanese army operation.
Any joint operation between the Lebanese army on the one hand, and Hezbollah and the Syrian army on the other would be politically sensitive in Lebanon and could jeopardize the sizeable US military aid the country receives.
Washington classifies the Iran-backed Hezbollah as a terrorist group.
In a recent speech, Hezbollah leader Sayyed Hassan Nasrallah said the Lebanese army would attack Islamic State from its side of the border, while Hezbollah and the Syrian army would simultaneously assault from the other side.
The Lebanese security source said the two offensives were not coordinated. "Each side is working alone," the source said. But a commander in the military alliance fighting in support of President Bashar al-Assad said that "naturally" there was coordination.
In a televised news conference, an army spokesman said the operation would continue until the army recovered control of Lebanese territory up to the border with Syria.
Last month, Hezbollah forced Nusra Front militants and Syrian rebels to leave nearby border strongholds in a joint operation with the Syrian army.
The Lebanese army did not take part in the July operation, but it has been gearing up to assault the Islamic State pocket in the same mountainous region.
Hundreds of IS fighters are estimated to be holed up in the enclave. "We started advancing at 5 am (0200 GMT)," the Lebanese source said. Footage broadcast by Hezbollah-run al-Manar TV showed the group's fighters armed with assault rifles climbing a steep hill in the western Qalamoun.
Lebanese President Michel Aoun was following the army operation, called "Jroud Dawn". "Jroud" refers to the barren, mountainous border area between Lebanon and Syria.
Hezbollah has provided critical military support to President Bashar al-Assad during Syria's six-year-long war. Its Lebanese critics oppose Hezbollah's role in the Syrian war.
Northeastern Lebanon was the scene of one of the worst spillovers of Syria's war into Lebanon in 2014, when Islamic State and Nusra Front militants attacked the town of Arsal.
The fate of nine Lebanese soldiers taken captive by Islamic State in 2014 remains unknown.
Shi'ite Hezbollah and its allies have been pressing the Lebanese state to normalize relations with Damascus, challenging Lebanon's official policy of neutrality toward the conflict next door.