SINGAPORE/KUPANG (EAST NUSA TENGGARA) - Uncertainty looms for thousands of passengers planning to travel to and from Bali as the island's Denpasar International Airport was shut after a volcanic eruption.

The airport in the Indonesian resort island was closed at 3am on Friday (June 29) due to the eruption of Mt Agung.

It reopened at 2.30pm, said Mr Israwadi, the corporate secretary of Indonesia's state-owned airport operator Angkasa Pura I that runs the airport. The move follows an evaluation revealing that the volcanic ash does not cover the airport's aerodrome and therefore, it is safe to host or welcome aircraft.

"The airport and navigation authority decide that airlines can still fly through alternative routes unaffected by the volcanic ash," Mr Israwadi told The Straits Times.

As of 2pm, the closure of the airport on Friday has caused the cancellation of 318 flights with 26,862 passengers scheduled on board. Of the overall flights, 203 are domestic flights, while 115 were international flights.

The airport operator advises passengers to and from Bali to check updates on their flights through respective carriers.

In Singapore, numerous flights from Changi Airport to Bali, as well as those from the Indonesian island, were cancelled or postponed as a result.

With the Bali airport reopening, all but two Bali-bound flights resumed from 3pm onwards, according to departure information on Changi Airport's website.

Singapore Airlines flights SQ938 and SQ942, originally scheduled for 8.20am and 9.15am respectively, were rescheduled to 3pm and 4pm. Coming in the other direction, flights SQ939 and SQ943, originally set for 2.35pm and 3.45pm arrivals, were rescheduled to 9.50pm and 10.30pm.

Many travellers had not known of the last-minute cancellations before arriving at Changi Airport for their flights.

"We came to the airport at 7am for our 9am flight to Bali, but saw the information on the (travel schedule) board," said German mechanical engineer Wolfgang Mueller, 60.

He and his wife, Mrs Laura Mueller, 55, have been in Singapore for three days, and were supposed to go to Bali for two weeks, on Singapore Airlines.

They were initially told by airline staff that the flight was postponed to 1pm, but were later told that a decision about the flight would be made at 2pm.

"If we can't fly, we will head back to Germany and, unfortunately, that would be the end of our holiday," said Mr Mueller.

Mrs Laura Mueller, 55, a pianist, said: "Everything in Bali has been booked and paid from three months back, so we are quite disappointed."

A check on the flight schedules showed that there are some cancellations for flights to Bali between 10.55am and 5pm.

When The Straits Times arrived at Changi Airport Terminal 2 at about 9am, a refreshments table with water, orange juice and pastries had been set up in front of the Singapore Airlines check-in counter for its affected passengers.

British tourists, Mr Nick Archer. 55. and his wife Anna, 35, said the booth was already there when they arrived at 7am for their 9am flight.

They, too, found out about the cancellation only upon reaching the airport.

"There's uncertainty now because we can only wait for our travel agent back home to inform us about what to do now," said Mr Archer.

He said that SIA offered them flights to alternative destinations, but he had to wait for his travel agent back in Britain, which has a seven-hour time difference with Singapore.

SIA staff onsite said passengers can choose to change their ticket to any other Asean destination, and the fee will be waived.

Should they choose to wait it out in Singapore on Friday night, the airline will help them get a room at Crowne Plaza Hotel, for free.

Ms Alicia Seah, director of public relations and communications at Dynasty Travel, said it currently has two groups of customers over in Bali. They are due to return on Sunday (July 1).

Another three groups will be going over in early July.

"Activities in Bali generally will continue as planned, but customers are advised to avoid going to areas near Mt Agung," said Ms Seah.

Calling the current situation "volatile" and one that "may change at short notice", Ms Seah said her company will help to rebook, reroute and refund travellers who are affected directly if necessary.

"But cancellation of tours is not expected. We will continue to monitor the situation closely and update our passengers," she added.

In Bali, on Thursday, as many as 8,334 passengers were affected by the eruption of Mount Agung as airlines AirAsia, Jetstar, Qantas and Virgin cancelled 48 flights from and to Ngurah Rai International Airport, which Denpasar International Airport is also known as.

Singaporean civil servant Taufiq Kasni, 27, learnt of the airport closure there from staff at the hostel he was staying at.

He is supposed to head to the airport at around 5pm to catch his flight back on Friday (June 29), and has been in Bali for 12 days to dive and surf.

"I'm a bit worried because I'm working tomorrow (June 30). If the airport remains closed and the flight is delayed until tomorrow, then I will have to inform my boss," he said.

When he received news of its reopening, he was glad that his wish for the airport's closure to last for just a few hours, came true.

"But now there's a traffic jam on the way there, because everyone wants to catch their flight out, in case we get stuck if the airport closes suddenly again," said Mr Taufiq.

Miss Victoria Goh, 24, who is in Bali with some friends for holiday, is flying back to Singapore on Saturday morning. She is not taking a return home for granted.

"Even though the airport is reopened now, anything can happen until our flight tomorrow, so we're keeping our fingers crossed that it remains open," she said.

Bali's highest mountain has spouted an ash column of 2,500m, but so far its status remains on alert. The active volcano hurled ashes 2km into the sky on Thursday (June 28).

Mount Agung last erupted late last year.

The volcanic ash affected some areas in the west and south-east west of the mountain, such as Purage and Besakih, and areas spanning 4km from the crater have been categorised as dangerous zones.

Residents living in the mountain slopes have evacuated independently, according to the agency's statement.

At least 309 people have taken refuge in three refugee centres in three villages in Karangasem Regency.

Mount Agung last had a major eruption in 1963, killing around 1,600 people. Its activity intensified last year, causing the evacuation of tens of thousands of people as well as reducing tourist arrivals to Indonesia's most popular tourism destination..

Indonesia, the world's biggest archipelago of more than 17,000 islands, sits on the Pacific's "Ring of Fire", making it vulnerable to earthquakes and volcanic eruptions.