Hezbollah seizes much of IS enclave on Syria-Lebanon border

Hezbollah has captured much of an Islamic State pocket on Syria's side of the border with Lebanon in a joint offensive with the Syrian army, its leader said on Thursday.

In parallel with the fighting, talks on a truce have begun with Islamic State but a military victory is more likely, Sayyed Hassan Nasrallah said in a televised speech.

Syrian troops and Iran-backed Hezbollah have been fighting to oust Islamic State from Syria's western Qalamoun region.

The attack began last week, coinciding with a Lebanese army offensive against Islamic State on its side of the border in northeast Lebanon.

The zone straddling the border is the last part of the Lebanese-Syrian frontier under militant control.

Both offensives have advanced towards the border from opposite sides. The Lebanese army says it is not coordinating the assault with the Syrian army or Hezbollah, which Washington classifies as a terrorist group.

Any joint operation between the Lebanese army on one hand and Hezbollah with the Syrian army on the other would be politically sensitive in Lebanon and could jeopardise the sizeable US military aid the country receives.

The frontier battle was nearing a "very big victory", Nasrallah said.

"So far, more than 270sq km have been fully captured on Syrian land" by Hezbollah and the Syrian army, he said. "Around 40 square kilometres remain under Daesh control."

Islamic State is on the back foot in Iraq and Syria. It has lost ground in Syria to various separate enemies over the past year and the eastern Deir Ezzor province its last major foothold.

Hezbollah has played a major role in fighting Sunni militants along the border, and has sent thousands of fighters into Syria to support President Bashar al-Assad's government against Syrian rebel groups.

Earlier this month, Nusra Front militants left Lebanon's border region under an evacuation deal after Hezbollah routed them in their last footholds there. Thousands of refugees also departed with them to rebel territory in Syria.

Northeast Lebanon saw one of the worst spillovers of Syria's war into Lebanon in 2014, when Islamic State and Nusra Front militants briefly overran the border town of Arsal.

The fate of nine Lebanese soldiers that Islamic State took captive then remains unknown.

Islamic State leaders in Syria's western Qalamoun had asked for negotiations, Nasrallah said on Thursday.

"The first condition of any deal reached with Daesh will be revealing the fate of the Lebanese soldiers," he added.

If the Lebanese state wanted to negotiate an evacuation deal with Islamic State militants on its own side, Damascus would be ready to cooperate, Nasrallah said.

"But the condition is an official Lebanese request, and public coordination, not under the table," he said.

Hezbollah and its allies have been pressing Lebanon to normalise relations with Damascus, challenging the state's policy of neutrality towards the conflict next door.

Hezbollah's role in the six-year Syrian conflict has drawn criticism from its Lebanese political opponents, including Sunni leader and prime minister, Saad al-Hariri.