An Indonesian volcano that has been rumbling for months has witnessed a major eruption, killing at least 16 people, officials say.

Among the dead on Mount Sinabung were a local television journalist and four high-school students and their teacher who were visiting the mountain to see the eruptions up close, Sutopo Purwo Nugroho, National Disaster Mitigation Agency spokesman, said.

Saturday's eruption happened just a day after authorities allowed thousands of villagers who had been evacuated to return to Sinabung's slopes, saying that activity was decreasing.

Three other bodies were recovered from the village later in the afternoon and were taken to hospital for identification, Nugroho said.

"We suspect there are more victims but we cannot recover them because the victims are in the path of the hot [ash] clouds," he said.

Television footage showed villages, farms and trees around the volcano in Sumatra province covered in thick grey ash.

Sinabung is still spitting clouds of gas and lava as high as 2,000 metres, and the number of dead could rise as the rescue efforts were being hampered by the darkness.

Following the eruption, all those who had been allowed to return home on Friday were ordered back into evacuation centres.

Three other people, a father and his son as well as another man, suffered from burn injuries, Johnson Tarigan, a Karo district official, said, adding that the three were in an intensive care unit of a local hospital.

Clouds of gas and lava

Sinabung had shown a reduction of activity since mid-January. However, several eruptions on Saturday sent lava and pyroclastic flows down the southern slopes up to 4.5km away.

Authorities have evacuated more than 30,000 people, housing them in cramped tents, schools and public buildings.

Many have been desperate to return to check on homes and farms, presenting a dilemma for the government.

Authorities on Friday allowed nearly 14,000 villagers to return home after they fled following previous eruptions.

The latest eruptions came just a week after President Susilo Bambang Yudhoyono visited displaced villagers in Sinabung and pledged to relocate them away from the mountain.

Sinabung is among about 130 active volcanoes in Indonesia and has sporadically erupted since September.

Its last major eruption was in August 2010, when it killed two people. Prior to that, it had been quite for four centuries.

In 2010, 324 people killed over two months when Indonesia's most volatile volcano, Mount Merapi, roared into life.

Source: Agencies