The epicentre of the quake was approximately 10.3 kilometres (6.4 miles) south of the major Turkish resort of Bodrum, a magnet for holidaymakers in the summer, and 16.2 kilometres east of the island of Kos in Greece, the US Geological Survey said.
It had a depth of 10 kilometres, USGS said.
Deprem sonrası Bodrum sokakları.. #deprem pic.twitter.com/9ii8tFKJjQ — Çağan (@CaganErkan) July 20, 2017
Two people were killed on the Greek island of Kos, the ANA news agency reported. The quake caused injury and damage, the agency cited the island's mayor as saying.
In the Turkish resort of Bodrum, television pictures showed throngs of worried residents and holidaymakers in the streets.
"The biggest problem at the moment are electricity cuts in certain areas (of the city)," Bodrum mayor Mehmet Kocadon told NTV television.
"There is light damage and no reports that anyone has been killed" in the area.
The governor of the southern Mugla province -- where Bodrum is located -- said some people had been slightly injured after falling out of windows in panic.
The quake was also felt on the Datca peninsula -- also a major resort area -- as well as Turkey's third city of Izmir on the Aegean to the north.
An AFP correspondent holidaying in Bodrum said the major earthquake had been followed by several aftershocks.
"The bed shook a lot. Some bottles fell and broke in the kitchen and the patio," said Turkish pensioner Dilber Arikan who has a summer house in the area.
"I screamed I was very scared because I was alone."
The quake was also felt by holidaymakers on the Greek island of Rhodes.
"We were very surprised. We were scared and we immediately went outside," Teddy Dijoux, who was holidaying with his family at a Rhodes resort, told AFP.
"That lasted a long time. I quickly gathered up my children to leave the hotel," said holidaymaker Sylvie Jannot.
Turkey and Greece sit on significant fault lines and have regularly been hit by earthquakes in recent years.
This year alone, Turkey's western Aegean coast was hit by several significant earthquakes, which brought back memories of past deadly earthquakes.
In June, a 6.3-magnitude earthquake gutted a village on the Greek island of Lesbos, killing a woman and leaving more than 15 injured. The quake also caused panic on Turkey's Aegean coast.
On August 17, 1999, a huge earthquake measuring more than 7.0 magnitude near the city of Izmit devastated vast areas in the country's densely populated northwestern zone, notably around Istanbul, killing over 17,000 people.
Kos adası #deprem Video @risvan3402 pic.twitter.com/PzmO1KiF8M — Şafak Başkılıç® (@SafakBaskilic) July 21, 2017
