* Bali airport website - Lombok airport website - (Recasts, adds fresh comment from airport officials, disaster agency, details)
DENPASAR, Indonesia, Nov 25 (Reuters) - Indonesian and regional authorities heightened flight warnings around Bali's Mount Agung on Sunday as the volcano's eruptions sent a plume of volcanic ash and steam more than 6,000 metres into the skies above the popular holiday island.
Ash from the eruption covered roads, cars and buildings near the volcano in the northeast of the island, while overnight a red glow of what appeared to be magma could be seen in photographs by Antara, the state news agency.
"Since last night the eruption has been a magmatic type eruption, not phreatic," Sutopo, a spokesman for Indonesia's Disaster Mitigation Agency (BNPB), said in a Twitter message, referring to the switch from a steam-driven eruption to one with magma.
"That's what has caused the eruption to continue to produce smoke and dark volcanic ash."
Vulcanologist Simon Carn said on his Twitter account: "Summit glow at #Agung indicates magma likely at or near surface. Satellites also detected thermal anomalies overnight."
Bali, famous for its surf, beaches and temples, attracted nearly 5 million visitors last year, but business has slumped in areas around the volcano since September when Agung's volcanic tremors began to increase.
Agung rises majestically over eastern Bali at a height of just over 3,000 metres. When it last erupted in 1963 it killed more than 1,000 people and razed several villages.
Australia's Bureau of Meteorology said in an advisory from it's Volcanic Ash Advisory Centre (VACC) in Darwin that the eruption was "expected to be ongoing.
Maps provided by VACC show an area of ash cloud heading southeast over the neighbouring island of Lombok, away from Bali's capital, Denpasar, where the main international airport is located.
Indonesia also upgraded its Volcano Observatory Notice for Aviation (VONA) to red, its highest warning, and said the ash-cloud top could reach 19,654 feet (6,142 metres) or higher.
However, officials said the airport would remain open for now as the ash could be avoided.
"The volcanic ash has only been detected in a certain area," the airport and other officials said in a joint statement.
All domestic flights and the airport itself were operating as "normal" and tests for ash had been negative, it said.
Yunus Suprayogi, general manager of Bali airport operator Angkasa Pura I, said food and entertainment would be provided as well as extra bus services if conditions changed and passenger numbers increased.
The airport would also "make it easier" for passengers to seek refunds and make other arrangements, he said, while noting that airlines had their own rules."
Carriers said they were assessing the situation on Sunday morning with several airlines including Qantas QAN.AX , Jetstar and Virgin VAH.AX resuming flights. ash tests came up positive, Lombok airport would be closed, airport officials said. Several flights to and from Lombok were cancelled on Sunday, according to the airport's website.
Indonesia's disaster agency has said Bali is "still safe" for tourists except for a 7.5-kilometre (4.7-mile) zone around Mount Agung.
"Despite the string of eruptions, there has not been an increase in volcanic activity," it said in a statement, noting that the emergency status for Agung remains at level 3, one below the highest.
An initial eruption on Tuesday prompted Singapore to update its travel advisory for the island, warning ash clouds could "severely disrupt air travel". Consulate in Denpasar warned citizens on Sunday to "be prepared for the possibility of being stranded" in Bali.