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.

The Indonesian authorities have now raised the alert to the highest level, fearing an eruption is imminent.

The alert was raised to level 4 just after 6am local time (22:00 GMT).

Ash has 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.

"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 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.

Bali's Mount Agung volcano eruption: in pictures Show all 22 1 /22 Bali's Mount Agung volcano eruption: in pictures Bali's Mount Agung volcano eruption: in pictures A fisherman drives a traditional boat as Mount Agung erupts AFP/Getty Bali's Mount Agung volcano eruption: in pictures Police evacuate a villager from inside the exclusion zone REUTERS Bali's Mount Agung volcano eruption: in pictures Villagers watch a river overflowing with water mixed with volcanic ash REUTERS Bali's Mount Agung volcano eruption: in pictures A flight information board shows cancelled flights at Ngurah Rai International Airport in Denpasar, Bali AP Bali's Mount Agung volcano eruption: in pictures A villager takes his cows to a field with Mount Agung volcano erupting in the background AP Bali's Mount Agung volcano eruption: in pictures Police officers wear masks as they patrol in a village in Karangasem AP Bali's Mount Agung volcano eruption: in pictures Clouds of ashes rise from the Mount Agung volcano AP Bali's Mount Agung volcano eruption: in pictures Passengers talk to airline workers as their flights are cancelled AP Bali's Mount Agung volcano eruption: in pictures Mount Agung AP Bali's Mount Agung volcano eruption: in pictures A boy takes pictures during Mount Agung's eruption AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano is seen spewing smoke and ash in Bali EMILIO KUZMA-FLOYD/via REUTERS Bali's Mount Agung volcano eruption: in pictures Balinese Hindus take part in a ceremony, where they pray near Mount Agung in hope of preventing a volcanic eruption AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano erupts for a second time in less than a week REUTERS Bali's Mount Agung volcano eruption: in pictures Balinese Hindus take part in a ceremony AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano is seen erupting from Glumpang village REUTERS Bali's Mount Agung volcano eruption: in pictures A Balinese farmer works as Mount Agung volcano spews volcanic ash EPA Bali's Mount Agung volcano eruption: in pictures Mount Agung during an eruption seen from Kubu sub-district in Karangasem Regency AFP Bali's Mount Agung volcano eruption: in pictures A villager walks near a rice field following the eruption of Mount Agung AP Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano spews volcanic ash Rex Features Bali's Mount Agung volcano eruption: in pictures An officer with the disaster management agency BPBD places a mask on child at a shelter for residents sheltering from Mount Agung volcano Antara Foto/Nyoman Budhiana/ via REUTERS Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano erupts REUTERS Bali's Mount Agung volcano eruption: in pictures Mount Agung's eruption is seen between Balinese temple at Kubu AFP/Getty Images

The warnings have forced the closure of the island's airport, stranding many holiday makers.

Earlier on Sunday officials had said the airport would remain open as as the ash could be avoided.

Virgin Australia 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.

Indonesia's disaster agency had said Bali is "still safe" for tourists except for a 7.5-kilometre (4.7-mile) zone around Mount Agung. The evacuation zone has subsequently been increased.

China's Consulate in Denpasar warned citizens on Sunday to "be prepared for the possibility of being stranded" in Bali.