Indonesia warned Tuesday its most active volcano could continue erupting for months as 50,000 remained in temporary shelters and airlines cancelled flights over the disaster-hit nation.

Searing grey fumes shot high into the sky and rolled down the slopes of the 2,914-metre (9616-foot) Mount Merapi six times after dawn, spreading fear and panic as the government issued an alert telling airlines to avoid the skies over central Java.

"It could go on for weeks, even months," government volcanologist Subandrio told AFP.

Almost 40 people have been killed since Merapi began erupting last week.

President Susilo Bambang Yudhoyono planned to visit the shelters on Wednesday, his spokesman Julian Aldrin Pasha said.

"He will evaluate himself the handling of the Merapi victims by officials. He also wants to receive reports on whether the local government has faced any obstacles there," Pasha said.

Seismic activity escalated dramatically last week at the volcano on the densely populated island of Java, with increasing lava spurts and about 500 multi-phased volcanic earthquakes recorded.

But thousands defied an order to evacuate farmland on its fertile slopes, deciding instead to stay at their homes and tend their animals.

Field coordinator Widi Sutikno, of the Sleman district on the southern slopes, said those in danger had all now left.

"All of them have realised the danger of Merapi," Sutikno said.

He said that some of the people in makeshift camps had been suffering from cold and respiratory illness.

"None of them suffer from any chronic illness so far," Sutikno said.

Malaysia-based AirAsia said it had cancelled four flights linking Kuala Lumpur to Yogyakarta and nearby Solo, while Silk Air cancelled two flights between Singapore and Solo.

"The flights... to Yogya and Solo have been cancelled only for November 2," an AirAsia spokeswoman said, referring to four flights to and from the cities. The company later said the flights would resume on Wednesday.

A statement from SilkAir, the regional wing of Singapore Airlines, said a flight from Singapore to Solo and its return leg were cancelled.

"Our next scheduled Singapore-Solo flight is on Thursday and we are closely monitoring the situation," the statement said.

Meanwhile the distribution of aid continued to Indonesia's Mentawai islands, where more than 400 people were killed by a tsunami last week, despite difficulty in reaching victims.

"We're working very hard to ensure that aid reaches every affected area, including remote ones," West Sumatra provincial disaster management official Joskamatir said.

"Sometimes relief dropped from the helicopters may get stuck on trees or fall into the sea, but we're continuously sending them," he said.

Indonesia straddles major tectonic fault lines and is part of a region known as the "Ring of Fire" that circles the Pacific Ocean.