No casualties were reported.

Clouds of hot ash shot one kilometre into the sky and an avalanche of lava and searing gas spilled down Mount Merapi's trembling slopes up to three kilometres at least six times since the morning as the volcano groaned and rumbled.

A series of strong pyroclastic flows were released from the actively growing lava dome in the inner summit crater of the 2968m-high volcano.

Pyroclastic flow is a volcanic phenomenon includes turbulent and hot avalanches of hot lava rocks, ash and volcanic gases mixed together.

She described the volcano's lava dome as growing rapidly, causing hot lava and gas clouds to flow down its slopes. Parts of the lava dome were collapsing, sending rocks and ash flowing down the southwest flank of the volcano.

Ash covered several villages and nearby towns, she said.

Mount Merapi has seen increased volcanic activity in recent weeks and ash plumes extended about 1.8km to the southwest of the volcano before dawn.

Indonesia's Volcanology and Geological Hazard Mitigation Center did not raise Merapi's alert status, which already was at the third-highest level since it began erupting last August.

Villagers living on Merapi's fertile slopes were advised to stay five kilometres from the crater's mouth and should be aware of the peril of lava, the agency said.

The volcano is on densely populated Java island near the ancient city of Yogyakarta. It is the most active of more than 120 active volcanoes in Indonesia and has repeatedly erupted with lava and gas clouds recently.

Merapi's last major eruption in 2010 killed 347 people.

Indonesia, an archipelago of 270 million people, is prone to earthquakes and volcanic activity because it sits along the Pacific "Ring of Fire," a horseshoe-shaped series of seismic fault lines around the ocean.

Australian Associated Press