The Lebanese army has launched an offensive against an enclave of the so-called Islamic State group on the northeastern border with Syria.
It comes as the Lebanese Shia group Hezbollah announced an assault on the militants from the Syrian side of the frontier.
The Lebanese army operation got underway at 3am Irish time, targeting IS 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.
A security source said the offensive was making advances with several hills taken in the push against the militants entrenched on fortified high ground, in outposts and in caves.
The operation by Hezbollah and the Syrian army targeted the area across the border in the western Qalamoun region of Syria.
Hezbollah-run al-Manar TV said that its fighters were ascending a series of strategic heights known as the Mosul Mountains that overlook several unofficial border crossings used by the militants.
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.
The Lebanese army said it was not coordinating the assault with Hezbollah or the Syrian army.
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 jeopardise the sizeable US military aid the country receives.
The United States classifies the Iran-backed Hezbollah as a terrorist group.
"There is no coordination, not with Hezbollah or the Syrian army," General Ali Kanso said in a televised news conference, adding that the army had started to tighten a siege of IS in th earea two weeks ago.
In a recent speech, Hezbollah leader Sayyed Hassan Nasrallah said the Lebanese army would attack IS from its side of the border, while Hezbollah and the Syrian army would simultaneously assault from the other side.
A commander in the military alliance fighting in support of Syrian President Bashar al-Assad said that "naturally" there was coordination between the operations.
