The volcano on densely-populated Java island was put on red alert on May 13, meaning it was considered poised to erupt. Its activity has been erratic since then but escalated again this week.

The vulcanology office in Yogyakarta, 30 kilometres (18 miles) south of the volcano, said Merapi sent a torrent of gas clouds some 4.5 kilometres down its southern slope early Thursday, the furthest the clouds have reached so far.

In the first six hours of Thursday, Merapi sent five other clouds to the southwest, reaching up to 3.5 kilometres, and 13 others southeast up to three kilometers.

None of the gas clouds reached inhabited areas.

The clouds, known technically as nuees ardentes but locally as "shaggy goats", consist of volcanic gases, ash and dust.

They can reach temperatures up to 500 degrees Celsius (930 degrees Fahrenheit) and speeds of hundreds of kilometres per hour.

Thirty-three lava flows also spewed from the crater in the first six hours of Thursday, streaming two kilometres down Merapi's flanks, the office said.

On Wednesday, the volcano emitted eight visible clouds of gas stretching three kilometres and 32 lava flows also reaching two kilometres.

Meanwhile officials said just over 18,000 people had left their homes on the higher slopes of the volcano by early Thursday.

District chief of Magelang in Central Java Singgih Sanyoto told ElShinta radio that some 11,500 people had been evacuated from Merapi's western flank.

The district disaster management centers in Klaten and in Sleman, Yogyakarta province, registered 3,507 and 3,084 evacuees each.

Many of the evacuees have moved voluntarily, jittery due to a deadly earthquake which rocked the region 11 days ago.

Experts fear that the mountain's growing lava dome could collapse, sending speeding clouds of searing gas down Merapi's slopes along with volcanic debris and lava.

The situation deteriorated this week after the collapse on Tuesday of a natural ridge which had so far helped contain larger lava flows.

One volcanologist warned Wednesday that a full moon due June 12 could hypothetically exert some influence on the dome if it reaches a critical condition by then.

The most significant torrents of volcanic gas prior to Thursday's were emitted on May 15, the last full moon.

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