Indonesia's Mount Merapi has been showing increasing activity, emitting large gas clouds and sending more than 15,000 villagers fleeing to safety.
The volcano has spewed out its largest gas cloud yet, experts said.
The Indonesian authorities are stepping up emergency preparations in case of a major eruption.
Scientists fear the volcano, which has been showing signs of erupting since early May, has been further destabilised by an earthquake nearby.
The quake, which struck near the ancient city of Yogyakarta two weeks ago, killed 6,200 people.
A major humanitarian operation is under way to help survivors, but local and international agencies are also aware they may have to face the possibility of a second disaster not far away, on the slopes of Merapi.
The volcano has been in what scientists call the early stages of eruption for weeks, sporadically belching out thick clouds of toxic gas, ash and red hot lava.
But there was a noticeable increase in its activity on Thursday, when it sent a cloud of hot gas 4.5 km (3 miles) down its southern slope, the furthest such clouds have reached so far.
The volcano also sent a series of other gas and ash clouds, as well as lava flows, streaming down the mountain in different directions during the course of the morning.
The BBC's Rachel Harvey, in Jakarta, says parts of the crater wall have collapsed, allowing lava to escape from different points and forcing the authorities to rethink which villages are now most at risk.
"A lot of people are panicking," said Sutomo, a government official in the area.
At least 15,000 people are reported to have fled from the southern and western flanks of the volcano on Thursday, heading to nearby towns.
Three weeks ago, the government raised the warning of an eruption on Merapi to the highest level of alert, and started evacuating people living near the crater to government-run camps.
But some people refused orders to leave, while others have chosen to return during daylight hours, citing the need to tend their crops and livestock.
This movement is making it extremely difficult for aid workers to get an accurate picture of how many people require help, according to the BBC correspondent in Indonesia, Rachel Harvey.