More than 25,000 people have fled their homes following a series of eruptions and lava flows from a volcano in Indonesia, an official said Sunday.

Mount Sinabung on the western island of Sumatra sent hot rocks and ash up to 5,000 metres (16,000 feet) in the air "several times" on Saturday, National Disaster Mitigation Agency emergency response director Tri Budiarto told AFP.

"So far, 25,516 people have been evacuated. There's nobody now within a five-kilometre (three-mile) radius of the crater. We are urging those living within seven kilometres southeast of the crater to move too," he added.

Hot lava, which has been spewing from the volcano for the past two weeks, has flowed into a river and filled up valleys with pyroclastic material, he said.

"There were small secondary explosions when lava flows came into contact with the water, but there are no casualties so far. We are urging people not to carry out any activity in the rivers," he added.

Mount Sinabung is one of 129 active volcanoes in Indonesia that straddle major tectonic fault lines, known as the Pacific Ring of Fire.

It had been quiet for around 400 years until it rumbled back to life in 2010, and again in September last year.

In August 2013, five people were killed and hundreds evacuated when a volcano on a small island in East Nusa Tenggara province erupted.

The country's most active volcano, Mount Merapi in central Java, killed more than 350 people in a series of eruptions in 2010.