Mount Sinabung in North Sumatra province unleashed volcanic ash as high as 8,000 meters (26,250 feet), the highest of its eruptions in recent days, said government volcano expert Surono.
The 2,600-meter (8,530-foot) -high mountain has sporadically erupted since September after being dormant for three years.
Officials raised the alert status of Sinabung to the second-highest level after an eruption early this month, prompting evacuation of more than 6,000 villagers living near its slopes. Its activities have continued since then, sometimes unleashing lava down the slopes.
"It was the strongest eruption in the recent days," said Surono, who like many Indonesians uses a single name.
Transportation Ministry spokesman Bambang Ervan said his office has issued a notice Monday for all airlines to avoid routes near the mountain.
Residents in Medan, the provincial capital about 50 kilometers (30 miles) northeastward, could see black smokes billowing from Sinabung.
Hours earlier, Mount Merapi, Indonesia most volatile volcano in Central Java, spewed volcanic ash about 2,000 meters (6,500 feet) into the sky, causing ash to fall in several towns.
National Disaster Management spokesman Sutopo Purwo Nugroho said about 600 families have gathered for a possible evacuation.
Merapi, which is in Yogyakarta province, killed more than 300 people and caused the evacuation of 20,000 villagers when it burst back to life in late 2010.
Indonesia which is located on the so-called Pacific "Ring of Fire," has 129 active volcanoes, making it prone to seismic upheaval.