Thousands of residents have been forced back into emergency shelters after a volcano in central Indonesia unleashed its most powerful eruption in weeks, Xinhua reports.

Mount Lokon, on Sulawesi, started erupting at around 10:35 local time on July 17. It spewed 3,500-metre ash and smoke into the air. There were no immediate reports of casualties.

The mount spitted hot lava, sand and rocks materials by up to over 500 meters horizontally, he said.

"This eruption is the biggest since last Sunday," said Kristianto.

Volcanologists have declared evacuation zone of 3.5 km and recommended evacuation.

Mount Lokon of 1,750 meters high is one of Indonesia's 129 active volcanoes. Its last major eruption in 1991 killed one Swiss hiker and displaced thousands of people.