Armed police forced tens of thousands of reluctant residents to leave the slopes of one of Indonesia's deadliest volcanos Friday amid warnings an eruption was imminent. The United Nations, meanwhile, mobilized hundreds of aid workers and medical supplies to the area.

Scientists raised the alert at Mount Kelud to the highest level earlier this week, pointing to rising temperatures in the lake of its crater and deep underground tremors. Authorities ordered 116,000 people living along the fertile slopes to evacuate, but many have refused, saying they need to tend to their crops and animals.

"If we didn't force them — in this case with a showing of firearms — the villagers would not budge," said local police chief Col. Tjuk Basuki, adding that residents have been repeatedly warned about the dangers of the volcano. "We had no choice but to do this for their safety."

Mount Kelud, located on densely populated Java island, last erupted in 1990, killing dozens. In 1919, a powerful explosion, heard hundreds of kilometers (miles) away, destroyed dozens of villages and killed at least 5,160 people.

Volcanic activity appeared to be stabilizing Friday, with no spikes in temperature and a reduction in the number, but not intensity, of underground tremors, said a senior government vulcanologist. But he noted that a similar pattern emerged days before the 1990 eruption.

"This is exactly the situation we are most afraid of," said Surono, who goes by only one name. "Last time, the volcano stabilized and ... then suddenly erupted. I can't tell what is going on inside Kelud ... but anything could happen anytime now."

Thousands of people have left the mountain, many settled in temporary shelters along its base. Some held Islamic prayers beneath a tent Friday, where a preacher told them to remain calm and follow the advice of authorities when the eruption finally comes.

"We pray to Allah to protect us always," Abdul Zukri told them.

The U.N. said in a statement Friday the World Health Organization had activated 100 medics, put 200 health facilities on alert and established 41 outreach health posts. Emergency health kits, masks and essential equipment also were distributed.

Indonesia, which has about 150 active volcanos spread across 17,500 islands, sits on the so-called Pacific "Ring of Fire" — a series of volcanos and fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.

Mount Kelud is 620 kilometers (385 miles) east of the capital, Jakarta.

The 1,731-meter (5,679-foot) volcano normally erupts without warning signs, like smoke or ash. Its explosive activity typically starts with a steam blast — when surfacing magma meets ground water — producing hot mud flows and pyroclastic surges.