BEIRUT, Aug 19 (Reuters) - The Lebanese army declared on Saturday that it had launched an offensive against an Islamic State enclave straddling the northeast border with Syria, a Lebanese security source said.

The army was targeting Islamic State positions near the town of Ras Baalbek with rockets, artillery and helicopters, the source said. It is the last part of the Lebanese-Syrian frontier under insurgent control. (Writing by Tom Perry; Editing by Richard Pullin)