The popular tourist destinations were rocked when a powerful earthquake struck the Aegean sea at 1.31am local time (11.30pm BST).
Two tourists, from Turkey and Sweden, were killed in Kos when a wall collapsed on to a bar.
A further five revellers were seriously injured and were flown to Crete by emergency services.
In total, around 100 people were hurt on the island, with a further 70 injured in nearby Bodrum.
Witnesses have reported seeing buildings in Kos shaking “like jelly”, and many slept on the beach after fleeing their homes and hotels.
Officials say that damage has been limited to old stone buildings and the island’s port.
“New buildings on the island, including numerous hotel units, are showing almost none or no problems,” said Yiannis Glynou, who leads the technical chamber in the Dodecanese islands.
A government spokesman said: “Slowly, slowly life is returning to normality,” adding that the damages are much less compared to those resulting from a 6.2 magnitude earthquake that hit Lesbos last month.
The earthquake has produced more than 160 aftershocks so far, the first of which measured 5.1 Richter. Around 60 of the subsequent tremors have been largely felt.
The European-Mediterranean Seismological Centre has created an map showing the locations of all of the aftershocks.
Professor Ethymios Lekkas, who heads Greece’s anti-seismic protection organisation, told newsit.gr that the tremors are “totally natural” and will continue for the foreseeable future.
He added: “It was a big earthquake … aftershocks are happening and will happen.”
The UK Foreign Office has advised holidaymakers in the affected areas to contact family and/or friends if it is safe to do so.
“You should be aware of the possibility of aftershocks, and follow the advice of the local authorities and/or your tour operator,” officials have advised.
“Flights and ferry services are being significantly disrupted as a result of the earthquake. Please contact your airline, ferry company or tour operator for updates to services.”
Turkey’s Disaster and Emergency Management Presidency (AFAD) head, Mehmet Halis Bilden, advised: “Our people should know that aftershocks are continuing, so they should refrain from entering damaged or vulnerable structures.”
