"The number of evacuees has risen to 5,956 people," said national disaster agency spokesman Sutopo Purwo Nugroho, adding they were mostly from six villages in a three-kilometre (1.9-mile) zone around the volcano.
The head of the North Sumatra disaster agency, Asren Nasution, said they had taken shelter in churches, mosques and government offices.
An AFP journalist at the scene said that some men who had fled ventured back to their villages to feed livestock but later returned to evacuation shelters.
Hendra Gunawan, a volcanologist at a local monitoring post, said the volcano was still spewing columns of smoke Monday but the activity was lower than the previous day.
The volcano was dormant for nearly 100 years before erupting in August and September 2010, forcing about 12,000 people to flee.
Indonesia has dozens of active volcanoes and straddles major tectonic fault lines known as the "Ring of Fire" between the Pacific and Indian oceans.
Last month five people were killed and hundreds evacuated when a volcano on a tiny 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 violent eruptions in 2010.