A 6.1-magnitude earthquake struck under the Mediterranean seabed east of the Greek island of Crete on Thursday, but local authorities said there were no immediate reports of injuries or damage.

The quake's epicentre was 20 kilometres (around 13 miles) below sea level, 50 km east of Crete on the Cretan-Rhodes ridge where the African tectonic plate meets the Eurasian plate, the US Geological Survey (USGS) said.

An aftershock took place a few minutes afterwards at the same location, the USGS said.

The Athens Observatory said the first tremor, which happened at 1807 GMT, struck some 14 km off the small island of Kassos and was strongly felt on nearby Crete.

Marinos Patakos, head of civil protection in Crete, told AFP that the quake sent people rushing into the streets in Crete's largest city Heraklion in the centre of the island, as well as in the east, "but for the moment there is no damage".

"I jumped up from my chair, the ground shook for at least five minutes," Aglaia Terzakis, who lives in the central Cretan village of Rotassi said.

"I did not see any material damage but it is nighttime and it's difficult to see what has happened," she added.

A doctor living in Kassos, which lies about 80 km northeast of Crete, said the quake was also strongly felt there and that people were "very concerned."

Greece is one of Europe's most earthquake-prone countries. In January 2014, several thousand people found themselves homeless on the holiday island of Cephalonia in the Ionian Islands after a 5.8 magnitude quake.