SINGAPORE - Even as Bali's Mount Agung threatens to erupt, airlines flying from Singapore to the Indonesian tourist island say they are continuing to monitor the situation, though flights are still operating according to schedule.

Indonesian national disaster management agency BNPB said on Monday (Sept 25) that the volcano has reached a "critical stage" with about 500 tremors - caused by magma rising closer to the surface - recorded a day, compared with just two in previous weeks.

The Indonesian authorities have already evacuated more than 48,000 of the 62,000 people living within a 12km exclusion zone around the crater of Mount Agung.

"We are closely monitoring the activity of Mount Agung in Bali and assessing flying conditions," said a Jetstar spokesman.

National carrier Singapore Airlines said it was also "closely monitoring" the situation in Bali.

Budget airline Scoot said those holding tickets purchased on or before Sept 22 2017, and are travelling between Sept 24 and Oct 2, can reschedule their flights to a later date with no charge.

Fewer than 100 customers have chosen this option, said a Scoot spokesman.

Jetstar spokesman Robin Goh said: "We have received only a handful of queries from customers to change their travel dates, which we have accommodated with no change fee."

Singaporean Monica Goh - who arrived for a five-day holiday in Bali with friends on Saturday (Sept 23), a day after the alert level was raised to the highest level - said she was not too worried about any possible eruption.

Seminyak, where she is staying, is about 70km away from Mount Agung.

However the 35-year-old, who works in the travel industry, had packed a bag with supplies such as a first-aid-kit just in case.

Ms Goh said she had a clear view of Mount Agung yesterday morning when she climbed Mount Batur, which is about 40km away.

Though there was "a little bit of smoke", it wasn't spewing out rapidly, she added.

"There's no widespread panic. Tourists are still going about their activities," she said.