A woman looks on as Mount Sinabung spews ash, as pictured from Sibintun village in Karo district, Indonesia's north Sumatra province, Nov 18, 2013. [Photo/Agencies]

JAKARTA - Indonesia on Monday issued a warning to international and domestic flights to avoid air routes near Mount Sinabung in North Sumatra which erupted Monday along with Mount Merapi in Central Java, officials said.

Mount Sinabung of 2,475 meters erupted Monday morning, unleashing a column of ash about 8,000 meter high, the biggest eruption since the volcano first rumbled back to life in September after being dormant for three years, said Agus Budianto, head of volcano observation at volcanology agency.

"This is the biggest eruption in vertical since September," he told Xinhua over phone.

The ash with volcanic material was spewed to southwest direction, said Agus.

Indonesian Transport Ministry Spokesman Bambang Ervan said that the volcanic ash was found by up to 25,000 meter high into the air that endangers flights.

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

The spokesman said that the routes were commonly passed by international and domestic flights, such as the flight from Australia.

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 on at 04: 53 am local time Monday, shooting a cloud of ash 2,000 meter high, Agus said. Tourists and travelers are banned from entering the slope of the volcano, he said.

The 2,911 meter high Mount Merapi's most violent eruption occurred in 2010 that killed about 350 people and displaced thousands others.

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