BLITAR, Indonesia - Continuous tremors rocked a threatening volcano on Indonesia's main Java island on Friday, a scientist said, warning it could erupt at any time.

Authorities placed historically deadly Mount Kelut on red alert more than two weeks ago, but until Thursday when hundreds of tremors shook the peak its activity had remained largely calm.

On Friday, the ongoing tremors had been continuous for more than three hours, volcanologist Agus Budianto said.

"The continuous tremors have been rocking the volcano for three-and-a-half hours, meaning that an eruption is imminent. This is the last key indicator we use before a volcano blows out," he warned.

"These tremors have continued for the longest period since the volcano was put on top alert," he told AFP.

Budianto said that volcanologists located in the volcano's monitoring centre 7.5 kilometres (five miles) from the crater were preparing to evacuate themselves.

The peak, about 90 kilometres from Indonesia's second largest city of Surabaya, was blanketed by thick fog and it was raining in the area, another volcanologist said.

Experts have said they expect an eruption of Kelut to consist of "heat clouds" of searing gas and volcanic debris rushing down the slopes, similar to the most recent 1990 eruption that claimed 34 lives.

Officials meanwhile were stepping up efforts to evacuate all residents -- approximately 130,000 according to Indonesia's health ministry -- within a 10-kilometre danger zone around the crater.

Many returned home despite evacuation efforts when the alert was initially raised, or refused to leave altogether.

Officials said they were planning to set up cinema screens and invite pop singers to perform at evacuation shelters in an effort to lure people to safety.

"We will set up an outdoor cinema at several shelters in Blitar district in order to attract people to evacuate," said Kamtono, a local official from one of the two districts that would be affected by an eruption.

"We are having big difficulties asking people to evacuate. They don't want to leave their livestock untended in the village and they also still believe the elderly people who are saying that Kelut won't erupt soon," he told AFP.

Kamtono said that some shelters would also be set up with stages where singers of "dangdut" -- a genre of Indonesian pop music -- would entertain people.

Indonesia sits on the so-called Pacific "Ring of Fire," where several continental plates collide, causing frequent seismic and volcanic activity.

The archipelago nation is home to 129 active volcanoes, two of which also began ominously spewing smoke last week.

More than 15,000 lives have been claimed since record-keeping began of Mount Kelut's eruptions, including an estimated 10,000 in a catastrophic 1586 eruption. A 1919 eruption spewed heat clouds that killed 5,160 people. - AFP/ir