The Lebanese army 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 on Saturday an assault from the Syria side of the border in the western Qalamoun mountain range.

The Lebanese army is targeting positions of the Islamic State of Iraq and the Levant (ISIL, known as ISIS) group, near the town of Ras Baalbek, with rockets, artillery and helicopters, a Lebanese security source.

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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 spokesman for the Lebanese army, Brigadier-General Ali Kanso, said the offensive did not start today.

"We have been planning the attacks for over two weeks now," Kanso said, speaking from the headquarters of the defence ministry.

"Our mission is to clear the border areas all the way to the Syrian borders."

Kanso said the advantage the army has against ISIL is the lack of air cover or tanks, but warned that ISIL fighters had good snipers and they know the area very well.

The Lebanese army said 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.

ISIL pocket

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 said ISIL controls an area of about 300sq km 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 Qusayr, which Hezbollah captured in 2013.

Meanwhile, Mamoun Abu-Nowar, a military expert and retired Jordanian Air Force general, told Al Jazeera that the Syrian government forces and Hezbollah already have the upper hand over ISIL fighters.

"It's a very tough battle. The terrain is quite difficult. It's a battle over who controlls the mountains, the hill tops and passages in between," Nowar said.

"But definitely ISIL would be wiped out from that area."