Indonesia's Mount Merapi unleashed a torrent of hot clouds, turning the sky a fiery red, as molten lava flowed down its slopes on Thursday with the eruption forcing over 250 people to flee their homes, authorities said.

The volcano -- one of the most active in the world -- erupted multiple times overnight, spewing gas, volcanic ash and rocks that flowed over five kilometres down its slopes, the head of Indonesia's National Disaster Management and Mitigation Agency said. 

"Due to the avalanches of hot clouds and ash, 253 people took shelter," Abdul Muhari said in a statement. 

Residents have been told to steer clear of an area spanning a seven-kilometre radius surrounding Merapi's crater over warnings of lava and airborne volcanic material, Muhari added. 

Hot clouds and smoke from the eruption blotted out the sky in parts of the densely-populated Java Island, near Indonesia's cultural capital Yogyakarta.

Mount Merapi is on its second-highest alert level since November 2020 after showing renewed activity.

Its last major eruption in 2010 killed more than 300 people and forced the evacuation of around 280,000 residents from surrounding areas.

That was its most powerful explosion since 1930, which killed around 1,300 people. An eruption in 1994 left about 60 people dead.

The Southeast Asian archipelago nation is home to nearly 130 active volcanoes.