DENPASAR, Indonesia (Reuters) – Indonesia raised its alert level for Bali’s Mount Agung volcano to the highest stage four on Monday and told residents to immediately evacuate areas near the summit, warning of the “imminent” risk of a larger eruption.

Flights were diverted away from the island’s airport due to the warning and the presence of volcanic ash from the erupting Agung.

“Continuing plumes of smoke are occasionally accompanied by explosive eruptions and the sound of weak blasts that can be heard up to 12 km (seven miles) from the peak,” the Disaster Mitigation Agency (BNPB) said in a statement.

“Rays of fire are increasingly visible from night to the following day. This indicates the potential for a larger eruption is imminent,” it said.

It warned residents in a 8-10 km (5-6 miles) radius around the volcano to evacuate immediately.

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 (9,800 feet). When it last erupted in 1963 it killed more than 1,000 people and razed several villages.

“In anticipation of the possibility and imminent risk of disaster, PVMBG (the Volcanology and Geological Disaster Mitigation Centre) raised Mount Agung alert level from three to four,” it said in a statement.

Bali’s airport has been closed as a result of the increased warning level, according to Agoes Soebagio, an official at the Transportation Ministry.

According to the Volcanic Ash Advisory Centre in nearby Darwin, Australia, there is “ash confirmed on the ground at Denpasar Airport” as well as ash at FL300 (which refers to flight level at 30,000 feet) in the vicinity of the volcano.

Soebagio said that Lombok Airport on nearby Lombok island had reopened after an earlier closure, because “no volcanic ash was detected”.

The alert level on Agung had been raised to the maximum in September, but was lowered in October when seismic activity calmed. However, volcanologists now say that the volcano has entered a new phase with magma now visible and meaning a greater risk of a large eruption.