JAKARTA (AFP) – The eruption of Indonesia's most active volcano Mount Merapi -- which has killed 151 people and displaced 320,000 -- has slowed, an official said Wednesday, though its activity remains high.

Mount Merapi, which means "Mountain of Fire", has been spewing ash and heat clouds since late October, killing people with torrents of boiling hot gas and rock and forcing hundreds of thousands to move to makeshift camps.

"The intensity of the eruption has decreased, but the volcano's activity is still high and it still emits heat clouds," government vulcanologist Surono told AFP.

"There was a burst of ash reaching up to 1,800 metres (6,000 feet) vertically last night, but the ash did not have the potential to reach Jakarta," he said.

Ash from the eruption seriously disrupted flights in and out of Indonesia at the weekend and could force US President Barack Obama to cut short his trip to the country.

Surono said data showed Merapi had belched more heat clouds at 5:30 am on Wednesday (2230 GMT Tuesday) which could reach up to four kilometres (two miles) from the volcano's crater.

The government announced a 20-kilometre "danger zones" from the top of the volcano on Friday, saying residents within the area should evacuate.