Hundreds of people have fled their villages along the slopes of Indonesia's most volatile volcano after it started belching smoke and ash, a disaster official said today.

Disaster mitigation agency spokesman Sutopo Purwo Nugroho said Mount Merapi on Java island rumbled as heavy rain fell around its cloud-covered crater early today.

He said the volcano unleashed a column of dark red volcanic material 1,000m high. The ash made the rain thick and muddy in several villages as terrified residents fled to safety.

He said the rumbling sound was heard 30km away, but an eruption did not occur and the volcano's alert level was not raised.

The 2,968m mountain is the most active of 500 Indonesian volcanoes. Its last major eruption in 2010 killed 347 people.