Indonesia’s most active volcano has erupted with its biggest lava flow in months, sending a river of lava and searing gas clouds flowing 3.5 kilometres down its slopes on the densely populated island of Java.

The rumbling sound could be heard several kilometres away as Mount Merapi erupted on Monday, sending hot ash 600 metres into the sky.

Ash blanketed nearby towns, but long-established evacuation orders are in place near the volcano, and no casualties were reported.

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It was Merapi’s biggest lava flow since authorities raised its danger level last November, said Hanik Humaida, the head of Yogyakarta’s Volcanology and Geological Hazard Mitigation Centre.

She said the lava dome just below Merapi’s south-west rim and the lava dome in the crater had both been active since the end of July.

The south-west rim dome volume was estimated at 1.8 million cubic metres and about three metres tall before partially collapsing on Monday morning, sending pyroclastic flows traveled fast down the south-west flank at least twice.

The 2968-metre peak is near Yogyakarta, an ancient city of several hundred thousand people embedded in a large metro area on the island of Java.

-AAP