JAKARTA - Indonesian officials have raised the alert level for the Mount Agung volcano on the island of Bali for the second time in less than a week.

The National Disaster Mitigation Agency raised the volcano alert from normal to vigilance last Thursday, and raised it again to the next level - standby alert - on Monday (Sept 18) night.

It also more than doubled the size of a no-go zone around the volcano, urging villagers and visitors to stay 6 km away from the crater, and up to 7.5 km away to the north, southeast and south-southwest.

Nearby residents have been told to prepare for evacuation in case the alert level is raised, reported Australian broadcaster ABC News.

The surrounding Karangasem district, an area of about 840 sq km, has a population of 408,000.

The Bali Provincial Regional Disaster Management Agency and Karangasem Regency have been told to develop contingency plans for the worst-possible scenario of an eruption, reported Antara news agency.

Volcanic tremor activity of Mount Agung, the highest mountain in Bali, has increased since early this month (Sept).

"Actually volcanic tremors have already happened since in the middle of August, 2017 but they then vanished before emerging again and since early in September it has continued to increase,"Gede Suantika, the head of volcanic mitigation of the Volcanology and Geological Disaster Mitigation Center of the Geological Agency of the Ministry of Energy and Mineral Resources, said on Sunday.

A Facebook video showing the rumbling of the volcano has since gone viral. Posted by trekkers last week, "the video was captioned "Activity at Mt. Agung today. Increasingly violent sulfur fumes and roaring sounds from the crater of Mt. Agung."

Records show that Mount Agung erupted in 1908, 1823, 1843, and 1963. The last eruption in March 1963 killed some 1,100 people and hurled an ash cloud as high as 10 km.