MATARAM, Indonesia (Reuters) - An earthquake of magnitude 6.2 struck off the Indonesian island of Lombok on Thursday, the United States Geological Survey said, but there were no immediate reports of casualties or damage.

The earthquake, which hit 234 km (145 miles) south of Lombok, at a depth of 29 km (18 miles), also briefly startled tourists on the Indonesian resort island of Bali.

The quake was strong, with tremors lasting for three to four seconds, but did not trigger a tsunami alert, said Sutopo Purwo Nugroho, a spokesman of Indonesia's Disaster Mitigation Agency (BNPB).

"The quake went on for quite a while," said a witness in Lombok. "People ran out from houses in panic."

(Reporting by Clarence Fernandez in SINGAPORE; Additional reporting by JAKARTA bureau; Editing by Kim Coghill)