The Lebanese army says it has launched an offensive against an ISIL enclave straddling the northeast border with Syria.
Simultaneously, Hezbollah, the Lebanese Shia group, and the Syrian army have announced an assault from the Syria side of the border in the western Qalamoun mountain range.
The Lebanese army is targeting positions of ISIL, or the Islamic State of Iraq and the Levant group, near the town of Ras Baalbek with rockets, artillery and helicopters, a Lebanese security source said on Saturday.
It is the last part of the Lebanese-Syrian frontier under ISIL control.
"In the name of Lebanon, in the name of kidnapped Lebanese soldiers, in the name of martyrs of the army, I announce that [this] operation ... has started," said army chief General Joseph Aoun.
The Lebanese army says it is not coordinating its military operations with the Syrian army.
"This is is the final area of the border between Lebanon and Syria which the Lebanese army says is still vulnerable to attacks by armed groups," Al Jazeera's Imtiaz Tyab, reporting from Beirut, said.
"They are hoping to push these armed groups out and that Lebanon will essentially secure its border completely, something which they haven't been able to do since the start of the Syrian conflict."
He said the area for the last few weeks had also been the site of a major military offensive which pushed out Jabhat Fateh al-Sham, the group formerly known as al-Nusra Front.
Lebanese President Michel Aoun was following the army operation, called Jroud Dawn.
Jroud refers to the barren, mountainous border area between Lebanon and Syria.
Last month, Hezbollah forced fighters belonging to Jabhat Fateh al-Sham and other Syrian opposition groups to leave nearby border strongholds in a joint operation with the Syrian army.
The Lebanese army, a major recipient of US military aid, did not take part in the July operation, but it has been gearing up to target the ISIL pocket in the same mountainous region.
A military source said about 500 ISIL fighters are holed up in the enclave.
Lebanese politicians say ISIL controls an area of about 300 square kilometers between the two countries, around half of which is in Lebanon.
The area stretches from the Lebanese town of Arsal and villages of Ras Baalbek and Qaa, to the outskirts of Syria's Qalamoun region and parts of the western Syrian town of Qusair, which Hezbollah captured in 2013.