Indonesia's Semeru volcano has erupted and the country's volcanology agency has increased the alert level of Java Island's tallest mountain to the highest.

The volcano spewed ash clouds as high as 2km above the mountain's peak, the agency said, adding residents should keep to a 2.5 km distance away due to risks.

Semeru, more than 3600 metres high, is one of Indonesia's 129 active volcanoes.

Java is the most densely populated island in Indonesia.

The eruption throughout the day blanketed several villages with falling ash and forced authorities to raise the volcano's alert level twice, from the third-highest level to the highest, Indonesia's Geology Agency said.

No casualties have been reported.

Indonesia straddles the "Pacific Ring of Fire," a highly seismically active zone, where dif

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