Hizbollah chief Hassan Nasrallah said he was ready to launch an offensive against ISIL on the Lebanon-Syria border, days after forcing Al Qaeda's former Syrian branch from the area.
In a televised speech broadcast on Hizbollah's Al Manar television channel on Friday, Mr Nasrallah also demanded news within days about the fate of nine Lebanese soldiers kidnapped by ISIL in 2014.
Mr Nasrallah said "eliminating" ISIL from the mountainous area known as Jurud along the Lebanon-Syria border "is in the interests of both Lebanon and Syria".
According to Mr Nasrallah, the Lebanese army will decide when to launch an offensive on ISIL, adding that "from the other side the Syrian army and Hizbollah are ready".
Mr Nasrallah said if the battle against ISIL is launched from both Syrian and Lebanese territory "that will lead to victory and be less costly for everyone".
"There is a final decision" to launch an offensive against ISIL, he said.
Addressing ISIL directly, he said: "The Lebanese and Syrians will come at you from all sides."
There was no immediate comment from the Lebanese army. But Lebanon's state news agency, NNA, reported on Saturday that Lebanon's military was "shelling heavily gunmen's areas in Jurud Ras Baalbek and Al Kaa".
Mr Nasrallah said that ISIL holds around 296 square kilometres on both sides of the border, of which 141 sq km are in eastern Lebanon.
His Shiite movement is a key ally of the Syrian government of president Bashar Al Assad and has been battling alongside regime forces since the onset of the conflict in 2011.
Mr Nasrallah's remarks come a day after nearly 8,000 Syrian refugees and jihadists from Al Qaeda's former Syrian branch were bused back to Syria following a ceasefire deal with Hizbollah.
In exchange, the jihadists released five Hizbollah fighters they had seized during clashes in Syria.
The swap was part of a broader ceasefire deal announced last week between the two sides which ended six days of fighting in the mountainous Jurud Arsal region in the restive border area.
The Lebanese army did not take part in the fighting between Hizbollah and the former Al Qaeda affiliate now known as Fateh Al Sham Front, but it set up defensive posts and reportedly co-ordinated with Hizbollah.
Mr Nasrallah on Friday said negotiations that led to the ceasefire with the jihadist group were possible after being approved by both the Lebanese and Syrian leaderships.