Airlines have been issued a “red warning” about the danger of volcanic ash in the skies close to Bali after Mount Agung emitted a thick plume of smoke reaching 4,000m (13,100 feet).
It is the second major emission from the Indonesian island volcano this week, and flights have been disrupted.
The red warning means an eruption is forecast to be imminent, with significant emission of ash likely.
Authorities have begun distributing masks in some areas as ash falls.
Bali is a major tourist destination, although the main resorts of Kuta and Seminyak are about 70km (43 miles) from the volcano.
The island’s main airport is for now operating normally, but some airlines have cancelled flights. Volcanic ash can damage plane engines.
Travelers to and from the region are being urged to contact their airline or travel agent to find out the status of their flight.