Some 6,000 people have already left the island, known for its spectacular cliffside views and a dormant volcano, which has been hit by hundreds of tremors since last week, officials said.

Another quake with a magnitude of 4.9 hit early on Tuesday morning amid other smaller tremors. No injuries or damage were reported on Santorini and the neighbouring islands of Anafi, Ios and Amorgos, which have also been affected by the seismic activity.

Experts admit they have not experienced something similar before, and cannot give a precise estimate on how long it will last.

"This is the first time this is happening, we have not seen it before," Athanassios Ganas, research director at the national observatory's institute of geodynamics, told state TV ERT.

He noted that the area had been hit with over 40 earthquakes with a magnitude of over 4.0 in the past 72 hours.

Santorini lies atop a volcano which last erupted in 1950 -- but an experts' committee on Monday said the current phenomenon was "not linked to volcanic activity."

According to the Greek coastguard, over 4,600 people have left the island by ferry since Sunday.

The country's leading air carrier Aegean Airlines said it had flown nearly 1,300 people out of Santorini on Monday, with another eight flights able to carry 1,400 passengers scheduled for Tuesday.

Schools on all four islands have been shut as a precaution until Friday.

The head of Greece's earthquake planning and protection authority, Efthymios Lekkas, has deemed a more threatening earthquake of 6.0 magnitude unlikely.

"The residents of Santorini should feel safe. There must not be panic," he told Mega TV on Tuesday.

Greek Prime Minister Kyriakos Mitsotakis on Monday also appealed for calm, whilst admitting that the phenomenon is "very intense".

Santorini attracted about 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.