"Kelud is in a critical phase. It could erupt now," Surono, head of Indonesia's centre for Vulcanology and Geological Hazard Mitigation, told Reuters by telephone from the capital Jakarta.

After the alert was first raised recently, more than 100,000 people were ordered to evacuate from around the volcano, 675 km (420 miles) east of the capital Jakarta but only 90 km (55 miles) southwest of Indonesia's second-largest city of Surabaya.

Indonesian officials were also closely monitoring three other volcanoes for increased activity.