More than 25,000 people continue to seek refuge in temporary camps after fleeing the slopes of Mount Talang on Sumatra, which has been spewing hot ash since Monday.

But Isya Nur Ahmad Dana of Indonesia's Vulcanology Office said that although there had been a rise in tremors in at least four of 11 peaks now under close watch, the chances of a volcanic explosion were slim.

"There has been increased activity in terms of volcanic seismicity but the energy is not enough to cause a major eruption," said Isya Nur Ahmad Dana of Indonesia's Vulcanology Office.

Seismologists have said the sudden volcanic outbursts may be due to a series of huge quakes off the coast of Sumatra, including the magnitude-9.3 shockwave that triggered last year's Indian Ocean tsunami that left 220,000 dead.

Another 8.7 quake three months later from the same geological faultline killed more than 670 people on the Sumatra coast island of Nias and fuelled fears that a third disaster was imminent.

But even as powerful new quakes continued to strike, vulcanologist Dana said there was little cause for concern.

"It's not too worrying at this stage," he told AFP.

Two earthquakes, one measuring up to 6.5 on the Richter scale and felt in Jakarta, shook Indonesia's Java island early Friday, but there were no casualties or damage.

The Meteorology and Geophysics Agency said the epicenter of the larger jolt was in the Sunda Strait, a stretch of water separating Java and Sumatra islands, in which the remains of the legendary Krakatoa continue to smoulder.

Anak Krakatau -- the "child" of the volcano that blew itself apart in 1883 in one of the worst natural disasters ever recorded -- was one of the peaks placed on alert earlier this week amid warnings of poisonous gas emissions.

Another tremor measuring 5.0 on the Richter scale was felt in the nearby city of Bandung, the capital of West Java province, the meteorology agency said. The epicenter was 19 kilometers (11 miles) south of the city.

A volcano near Bandung, Tangkuban Perahu, has also been put on alert after an increase in activity, but authorities say there is no need to evacuate any of the 7.5 million people living in the densely-populated area in its shadow.

Bandung next week will play host more than 50 heads of state, including Chinese President Hu Jintao and Japan's Junichiro Koizumi at a summit commemorating the 50th anniversary of the Asia-Africa Conference.

Late Thursday, an "intense" tremor registering 5.8 was recorded about 150 kilometres southwest of the Sumatran coastal city of Padang, which was gripped by quake-induced panic earlier this week as thousands fled fearing a tsunami.

Indonesia's President Susilo Bambang Yudhoyono visited Padang and the nearby Mount Talang volcano on Wednesday to urge residents to remain calm in the face of terrifying seismic events.

The archipelago nation has nearly 130 active volcanoes, forming part of the Pacific "Ring of Fire" -- an arc of intense seismic activity that stretches from quake-prone Japan through Southeast Asia and across the Pacific basin.

Its proximity to the junction of three continental plates, which jostle under immense pressure, makes it particularly vulnerable to earthquakes and eruptions.