More than 22,000 people have been displaced by the volcano's latest round of activity, which began in September, the National Disaster Management Agency said.
"It still has a lot of energy, so it's expected to continue to erupt for some time," said disaster management agency spokesman Sutopo Nugroho.
He said the volcano has erupted more than 20 times since Tuesday.
Many of the refugees are in poor conditions in government buildings and makeshift shelters in the Karo district of North Sumatra province, aid workers said.
At least seven people have died from various causes, including high blood pressure and respiratory problems, said Riska Siregar, who works for a local organisation providing aid to those who fled their villages.
Scores of others needed medical treatment, she said.
"Many displaced people are depressed because they have been away from their homes for weeks with no end in sight to their plight," she said.
Ms Nugroho said there were enough food supplies for the refugees for the next few days.
The 2,460-metre volcano had been dormant for 400 years before it erupted in August 2010.
There are nearly 130 active volcanoes across the Indonesian archipelago.