Indonesian authorities raised the danger level for the volatile Mount Merapi volcano on the densely populated island of Java on Thursday and ordered a halt to tourism and mining activities.
Indonesia's geological agency raised Merapi's alert level, which had been at the third-highest level since it began erupting last year, to the second-highest level after sensors picked up increasing activity.
"This condition can trigger a magma extrusion process or an explosive eruption,'' National Disaster Mitigation Agency spokesman Raditya Jati said in a statement.
He said authorities have halted the climbing of Merapi and mining activities along its rivers. Only disaster agency personnel and researchers will be allowed to enter the restricted area.
Merapi spewed ash and hot gas in a column as high as 6 kilometers (3.7 miles) into the sky in June, but no casualties were reported.
Its last major eruption in 2010 killed 347 people and caused the evacuation of 20,000 villagers.
The 2,968-meter (9,737-foot) mountain is about 30 kilometers (18 miles) from the Yogyakarta city center. About a quarter million people live within a 10-kilometer (6-mile) radius of the volcano, according to authorities in surrounding districts.
The head of Yogyakarta's Volcanology and Geological Hazard Mitigation Center, Hanik Humaida, said villagers living on Merapi's fertile slopes are advised to stay 5 kilometers (3 miles) from the crater's mouth.
Indonesia, an archipelago of more than 250 million people, sits on the Pacific "Ring of Fire'' and is prone to earthquakes and volcanic eruptions. Government seismologists monitor more than 120 active volcanoes.