The tremor struck shortly after 0300 on April 4, about 300km (186 miles) south-west of Cilacap at a depth of 10km, the agency said.

Thousands of people poured out of their homes and ran to high ground, witnesses told an Indonesian radio station.

There have so far been no reports of any casualties or damage. The BMKG's technical chief said the shaking from Monday's earthquake had been felt in the Pangandaran and Cilacap districts of central Java.

"This quake roused people from their sleep," said Suharjono. Those who received the tsunami alert SMS message from the agency were asked to warn others of the potential danger. State energy firm Pertamina said its refinery in Cilacap was not damaged.

Indonesia is located on the Pacific "Ring of Fire", an arc of volcanoes and fault lines encircling the Pacific Basin.