JAKARTA (REUTERS) - An Indonesian volcano spewed a giant ash cloud 5km into the sky on Monday (Aug 10) in its second eruption in three days, emitting a thunderous noise and turning the sky dark, the authorities and witnesses said.

The eruption of Mount Sinabung on the island of Sumatra comes after more than a year of inactivity and was the second since last Saturday, as the authorities warned residents and tourists about possible lava flows.

Dramatic footage of the morning eruption captured by residents showed a giant cloud of thick ash rising from the peak of the 2,460m mountain in Karo, North Sumatra.

"The sound was like thunder, it lasted for less than 30 seconds," resident Fachrur Rozi Pasi told Reuters by phone.

Residents have been advised to stay outside of a 3km radius of the volcano and to wear masks to minimise the effects of falling volcanic ash, the volcanology agency said in a statement.

No casualties have been reported and a spokesman for the civil aviation authority said flights were still operating in the region.

"The situation around Mount Sinabung is very dark now," said Mr Gilbert Sembiring, who was visiting a friend in Naman Teran Kampung when Sinabung erupted.

"It was bigger than the eruption a couple of days ago."

Located in one of the world's most volcanically active countries, Sinabung had been inactive for centuries before it erupted again in 2010.