Jakarta, Nov 18 (IANS): Indonesia issued a warning to international and domestic flights to avoid air routes near Mount Sinabung and Mount Merapi, both of which erupted Monday, officials said.

The 2,475-metre-high Mount Sinabung, located in North Sumatra province, erupted Monday morning, shooting a column of ash about 8,000 metres high, the biggest eruption since the volcano first rumbled back to life in September after being dormant for three years, Xinhua quoted Agus Budianto, head of volcano observation at the country's volcanology agency, as saying.

"This is the biggest eruption in vertical since September," Agus Budianto told Xinhua.

The ash with volcanic material spewed towards the southwest direction, Budianto said.

Indonesian transport ministry spokesman Bambang Ervan said the volcanic ash was found to have shot up to 25,000 metres in the air and that would endanger flights.

"This (Monday) morning, we issued a warning to flights to avoid the air routes W11, W12, L774 and R456 near Sinabung volcano," he told Xinhua.

The spokesman said the routes were commonly used by international and domestic flights.

Over 6,000 people have been displaced since the beginning of this month as Mount Sinabung erupted intermittently.

Mount Merapi, the most active volcano in Indonesia and located in central Java, erupted earlier Monday, shooting a cloud of black ash about 2,000 metres high, Budianto said.

The 2,911-metre high Mount Merapi's most violent eruption was in 2010, in which at least 350 people were killed and thousands displaced.

Indonesia, a vast archipelago country with 17,500 islands, is home to 129 active volcanoes and sits on a vulnerable quake-prone zone called the Pacific Ring of Fire.