Bali's largest volcano, Mount Agung, could erupt within "a matter of hours", geologists have warned.

Indonesia's volcanology centre said on Tuesday (26 September) that the volcano has entered a "critical phase" and that an eruption is imminent.

Almost 60,000 people have been evacuated from the area surrounding Mount Agung in recent days after hundreds of tremors and signs of magma rising to the surface of the volcano were reported. Geologists said the volcano is experiencing unprecedented levels of seismic activity.

"Instrumentally we have never recorded such high energy or seismicity from Mount Agung," Devy Kamil Syahbana, a seismologist from Indonesia's centre for volcanology, told The Guardian.

Mount Agung experienced 849 volcanic earthquakes on Monday (25 September) and between 300 and 400 earthquakes by midday on Tuesday, according to the volcanology centre.

"We need to pay attention because these kinds of earthquakes indicate the movement of magma and increase the probability of an eruption," Syahbana said.

Indonesia's national disaster agency spokesperson, Sutopo Purwo Nugroho, said there is a "definite possibility" that the earthquake will erupt, but that it is unclear when it will happen.

Thousands of people have been moved to temporary shelters in town halls and schools. Indonesian President Joko Widodo is expected to visit evacuees later on Tuesday.

Heightened volcanic activity was first noticed at Mount Agung in August and officials have been making arrangements for mass evacuation since then.

Several countries, including the US, UK, Australia and Singapore, have warned their citizens against travelling to Indonesia. Airlines are continuing to operate flights to and from Bali as normal.

Mount Agung, located more than 3,000m above sea level in the eastern part of Bali, is a popular tourist destination, situated around 70km from the main tourists areas of Kuta and Seminyak.

Indonesia, which is home to nearly 130 active volcanoes, lost more than 1,000 people in 1963 when Mount Agung last erupted.