Speaking from Brussels, Mitsotakis said authorities were monitoring a "very intense" geological phenomenon over recent days, adding: "I want to ask our islanders above all to remain calm."

Known for its spectacular cliffside views and a dormant volcano, Santorini and neighbouring Aegean Sea islands have been hit by hundreds of tremors since last week, the largest with a magnitude of 4.9.

The neighbouring islands of Anafi, Ios and Amorgos have also been affected.

An experts' committee on Monday said some 200 tremors over 3.0 magnitude had been recorded since Saturday.

The committee stressed that the phenomenon was "not linked to volcanic activity."

The quakes prompted authorities to send rescue units to the area and close schools Monday. Some areas have been declared off-limits because of rockslide risks.

On Monday, it was decided to shut schools on the four islands until Friday.

Britain and France issued travel warnings, urging their nationals to follow the advice of local officials.

Greek authorities have also asked people to avoid large gatherings in enclosed spaces and stay away from certain ports and derelict buildings. They are also urged the emptying of swimming pools.

Efthymios Lekkas, president of the Organisation of Antiseismic Planning and Protection, told public broadcaster ERT on Monday that there was a "faint possibility of a 5.5-magnitude earthquake", but ruled out one measuring over six.

Many among Santorini's population of over 15,000 spent the night outdoors, while hundreds of permanent residents are leaving the island by sea and air.

- 'This feels different' -

Kostas Sakavaras, a tourist guide who has lived on the island for 17 years, told AFP he had never experienced this level of seismic activity before.

"It was shaking every three to four hours yesterday. This feels different from the other times," he said.

Sakavaras said he left the island on Sunday with his wife and two children, on a ferry that was full.

"We plan to stay (on the mainland) until the end of the week. I think it's going to escalate tomorrow and I hope then it will calm down," he said.

A Greek coastguard source said over a thousand people had left the island by sea on Sunday. Around a thousand more boarded a ferry on Monday evening, AFP reporters said.

Aegean Airlines, Greece's biggest carrier, said it had dedicated four additional flights to and from Santorini, and two on Tuesday after a request from the country's civil protection ministry.

Operator Sky Express also said it had added two extra flights on Monday and Tuesday.

Ferry operator Attica Group said it was sending an additional ship late Monday and could contribute more if needed.

"We are worried, we all want to leave... mainly for the children," said Dimitris Selistai, a 45-year-old labourer queueing outside a ferry ticket office.

Santorini is among Greece's premier travel destinations, with 3.4 million visitors in 2023.

European travel agents contacted by AFP said foreign visitors to Santorini at this time of year were minimal, with bookings expected in the spring.

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Aegean Airlines