Mount Sinabung spews volcanic materials into the sky, as seen from Tanah Karo, in Indonesia's North Sumatra province, on Friday. (The Associated Press)

An Indonesian volcano that was quiet for four centuries shot a new, powerful burst of hot ash more than three kilometres in the air Friday, sending frightened residents fleeing to safety for the second time this week.

The force of the eruption — the strongest so far — could be felt eight kilometres from the volcano in the North Sumatra province.

"This was a big one," said 37-year-old Anto Sembiring, still shaken after abandoning his coffee shop in the middle of the danger zone. "We all ran as fast as we could. … Everyone was panicking."

The eruption of Mount Sinabung on Sunday and Monday — which caught many scientists off guard — forced more than 30,000 people living along its fertile slopes to evacuate to cramped emergency shelters in nearby towns.

Wearing surgical masks to protect themselves from the smoky air, many have complained about the steadily deteriorating conditions, from poor sanitation and short food supplies to having to sleep on hard, cold floors.

"It's especially bad for the kids," said Yacoubus Runtuwene, a World Vision relief worker. "We're staring to see a lot of respiratory problems, diarrhea and eye irritations."

Tired of waiting, thousands of people started returning to the mountainside earlier this week so they could clean up their soot-covered homes and salvage what they could from their vegetable farms and rice paddies.

But several hours before Friday's pre-dawn blast, a new alert was issued. Some people trudged back down the slopes, carrying blankets, clothes and food. A handful of others insisted on staying, even after the new explosion, which caused the entire mountain to tremble violently for five minutes.