Beirut: The Lebanese army has launched an offensive against an Islamic State enclave on the northeast border with Syria, a Lebanese security source said, as Hezbollah and the Syrian army announced an assault from the Syrian side of the border.

The Lebanese army was targeting Islamic State positions near the town of Ras Baalbek with rockets, artillery and helicopters, the source said.

Hezbollah fighters stand near a four-wheel motorcycle near the Lebanon-Syria border. Credit:AP

The area is the last part of the Lebanese-Syrian frontier under insurgent control.

"We started advancing at 5 am (1200 AEST)," the Lebanese source said.