In the early hours of Tuesday, Merapi sent out 10 heat clouds and 120 lava trails, some of them two kilometers (one mile) long, said Tri Yani of the vulcanology office in Yogyakarta.

Plumes of smoke were seen rising some 900 metres (3,000 feet) into the air -- nearly double the height seen the previous day. The area remains on high alert for a possible eruption.

Scientists have warned that although the magma flow which forms a dome at the peak appears to be weakening, the structure may collapse and spew out millions of cubic metres of volcanic rock and lava.

After calming down for a few days, Merapi belched significant heat cloud torrents shortly after Saturday's strong earthquake, which killed more than 5,400 people in central Java.

On Monday, Merapi sent out 186 lava trails and 88 heat clouds, Yani said.

The vulcanology office also recorded 57 aftershocks on Monday in the wake of Saturday's earthquake, three of them measuring more than 4.0 on the Richter scale.

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