Indonesia's Mount Semeru volcano erupts, top alert status triggered

Indonesia's Mount Semeru erupted, spewing hot ash clouds a mile into the sky and sending rivers of lava down its side, prompting authorities to raise the alert status to the highest level.

The eruption of the highest mountain on Indonesia's main island of Java, around 800 kilometres southeast of the capital, Jakarta, sparked evacuations of nearby villages exactly one year after its last major eruption killed dozens of people.

The increased threat level "means the danger has threatened the people's settlement and the volcano's activity has escalated".

"Hot avalanches" caused by piles of lava at the tip of the 3,676-metre volcano slid down after the eruption, National Disaster Mitigation Agency spokesperson Abdul Muhari said in a statement.

No casualties or injuries were reported immediately after the eruption but Gunawan warned nearby residents not to travel within eight kilometres of the crater after the threat level was raised to four.

Images on local TV showed evacuees, mostly women and children, taking shelter in a school.

Videos showed a huge black cloud rising from the volcano's crater, engulfing the sky and blocking the sun in nearby villages.

The villages were being battered by monsoon rains by the afternoon and the rainfall was mixing with volcanic ash.

Residents were also told to avoid a southeastern area 13 kilometres along a river in the direction where the ash was travelling.