Nasrallah also demand news within days about the fate of nine Lebanese soldiers kidnapped by IS in 2014
Hezbollah chief Hassan Nasrallah said on Friday he was ready to launch an offensive against the Islamic State group on the Lebanon-Syria border, days after forcing al-Qaeda's former Syrian branch from the area.
In a speech broadcast on Hezbollah's Al-Manar television channel, Nasrallah also demanded news within days about the fate of nine Lebanese soldiers kidnapped by IS in 2014.
Nasrallah said "eliminating" IS from the mountainous area known as Jurud along the Lebanon-Syria border "is in the interests of both Lebanon and Syria".
According to Nasrallah, the Lebanese army will decide when to launch an offensive on IS, adding that "from the other side the Syrian army and Hezbollah are ready".
Nasrallah said if the battle against IS 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 IS, he said.
Addressing IS directly, he said: "The Lebanese and Syrians will come at you from all sides."
There was no immediate comment from the Lebanese army.
Nasrallah said that IS holds around 296 square kilometres (115 square miles) 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.
Nasrallah's remarks come a day after nearly 8,000 Syrian refugees and militants from al-Qaeda's former Syrian branch were bused back to Syria following a ceasefire deal with Hezbollah.
In exchange, the militants released five Hezbollah fighters they had seized during clashes in Syria.
The swap was part of a broader cease-fire 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 Hezbollah and the former al-Qaeda affiliate now known as Fateh al-Sham Front, but it reportedly coordinated with Hezbollah.
Nasrallah on Friday said negotiations that led to the ceasefire with the militant group were possible after being approved by both the Lebanese and Syrian leaderships.
Meanwhile, the Lebanese army said on Saturday it will not coordinate with the Syrian army to fight against Islamic State in the Lebanese-Syrian border zone, a military source told Reuters on Saturday, rejecting a local media report of direct military cooperation between the two.
The source said the Lebanese army had the military capability to confront and defeat the group without any regional or international support.
The presence of Islamic State and Nusra Front militants in pockets on Lebanon's border is the biggest military spillover into the country from Syria's civil war.
An offensive launched last month by Lebanon's Hezbollah - a key ally of Syrian President Bashar al-Assad - forced Nusra Front militants to leave for a rebel-held area in northwest Syria under an evacuation deal.
The Lebanese army did not take part in that offensive, but has been widely expected to lead an attack against the Islamic State pocket.