Indonesia issued a red alert after the Sinabung volcano erupted, blanketing the area in thick and acrid black smoke, disaster officials said, although no casualties have yet been reported.

"It's clearly dangerous so we've raised the warning to the highest level, or red level," said Surono, head of the nation's volcano disaster alert centre.

"From the crater, it shot smoke and volcanic ash 1,500 metres (5,000 feet) into the sky," he told AFP.

The 2,460-metre (8,100-foot) Sinabung in northern Sumatra has not erupted for more than 400 years but had shown "some volcanic activity" since Friday, Surono said, adding that they were monitoring the situation.

Villagers said they saw lava emerge from the crater around midnight, about 15 minutes before the eruption.

"I saw flames flickering, very red right at the top. Previously there was only smoke," Terkilin Sembiring was quoted as saying by Detikcom news website.

"At the time, we heard a sound like an aircraft flying past. We thought it was a government official's plane but it turned out that the sound came from the volcano," another villager Maslin Pandia was quoted as saying by Kompas.com news website.

Television footage showed black smoke shooting up into the sky and lava overflowing from the crater as residents fled the area in pickup trucks and cars.

More than 18,000 people have been evacuated from several affected villages to towns outside a six-kilometre "danger zone", officials said.

"The ash has spread to a distance of 30 kilometres (20 miles) from the volcano. Many of the villagers evacuated were farmers and they said the ash had settled on their vegetable farms," search and rescue team official Mohammad Agus Wibisono told AFP.

There were however no flight disruptions, Wibisono said.

Disaster Management Agency spokesman Priyadi Kardono told AFP that many residents in four affected villages at the foot of the volcano had fled immediately after the eruption.

"Many had left their homes even before they were evacuated. They said the volcano was spewing thick black smoke, small stones and sulphur. They were so scared they decided to leave their homes and go to the city," Kardono said, adding that a rescue team had been sent to survey the area.

"The area is blanketed with thick smoke and there's a strong smell of sulphur," he added.

Kardono said while there were no reports yet of any deaths or injuries, "many" had reported breathing difficulties.

"We have anticipated that people may have respiratory problems from inhaling dust. So we've given them face masks and are preparing to send in medicines for respiratory infections," he added.

The red alert would likely remain for "at least a week" as vulcanologists monitor the situation, Kardono said.

"There's little data on Mount Sinabung. The eruption took the experts by surprise. We don't know when it might erupt again so, it's best for people to stay away until the experts can determine when it's safe to go home," he added.

Antara news agency quoted local police as saying that two people had died from heart attacks. Karo district official Andes Mbaga earlier said one of them had "breathing problems which could have been worsened by the ash and dust floating around".

Evacuees from 18 affected villages were "all doing well" and those who complained of breathing problems were receiving medical help.

"Villagers are too traumatised to go home but the situation is under control, we have enough tents, food and water," he said, although volcanic activity had reduced significantly.

"We're going to stay put here. Mount Sinabung has never spouted so much smoke and at such a high level. We're so scared that it might erupt," villager Edi Ginting told Jawa Pos news website.

Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity. It has more active volcanoes than any other country.

Earlier this month, four people went missing after the 1,784-metre Mount Karangetang, on the remote island of Siau in North Sulawesi province, erupted.

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