DENPASAR, Indonesia (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 covered roads, cars and buildings near the volcano in the northeast of the island, while scores of flights were cancelled and overnight a red glow of what appeared to be magma could be seen in photographs by Antara, the state news agency.  The activity of Mount Agung has entered the magmatic eruption phase. It is still spewing ash at the moment but we need to monitor and be cautious over the possibility of a strong, explosive eruption,  said Gede Suantika, an official at the volcanology and geological disaster mitigation agency. 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 s Volcanic Ash Advisory Centre (VACC) in Darwin issued maps showing an 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. After resuming flights on Sunday morning, Virgin Australia again cancelled flights on Sunday afternoon following a change in the aviation colour code from orange to red.   Due to the significant volcanic ash and current weather conditions, we have made the decision to cancel the rest of today s flights to and from Bali as a precautionary measure,  Virgin said in a statement on its website. AirAsia also cancelled its remaining flights to Bali and Lombok. Qantas and Jetstar flights were continuing as of Sunday afternoon but Jetstar warned on its website that flights could be subject to change at short notice for safety reasons. Indonesia s flag carrier Garuda said it was cancelling all flights to and from Lombok.  Lombok airport was closed late on Sunday afternoon and authorities would assess the situation in the morning, a transport ministry spokesman said.  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. China s Consulate in Denpasar warned citizens on Sunday to  be prepared for the possibility of being stranded  in Bali. 