"Although visually one cannot see it because of the clouds, (Mount) Semeru is continuously erupting every 15 minutes or so, spewing ash and heated gas clouds," said Herry, from the vulcanology observation station in Lumajang, on the northern slope of the mountain.

Herry said the activity was part of the volcano's normal pattern of behaviour and there was no need to raise its alert status.

Ash spewed from the crater of the 3,676-meter (12,130-foot) volcano has blanketed several areas around the slopes in the past few days, he said.

The volcano last erupted in February 1994, killing nine people.

Meanwhile, authorities further downgraded the alert status of Central Java's Mount Merapi by another rung to "vigilance" on Monday morning, said Sapari, from the vulcanology office in Yogyakarta, a city just south of the volcano.

"Merapi has not emmited heat clouds for the past three weeks," he said.

The warning is now one step above "normal" but two below the peak alert, which scientists issue when they fear an eruption could occur within 24 hours.

Merapi was placed on top alert on May 13 and remained there for nearly the entire next two months. Two people sheltering in an emergency bunker were killed when ash and volcanic debris smothered them.

Authorities also downgraded the top alert status on a third volcano, Mount Karangetang on North Sulawesi's Siau island, on Saturday. Scientists said last month they feared an eruption when lava began oozing from its crater.

About 4,000 villagers were initially evacuated from Karangetang's slopes.

The archipelagic nation of Indonesia sits on the "Pacific Ring of Fire", where continental plates meet and cause frequent volcanic and seismic activity.

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