A powerful 6.8-magnitude earthquake struck off Greece on Friday and was felt strongly in the tourist hotspot island of Zante, monitoring agencies said.

Electricity in the main town, also called Zante, was down while the fire service reported damage to the road network caused by landslides.

It hit off the southern part of the island, also known as Zakynthos, in the Ionian Sea at 1.50 am (2250 GMT Thursday), according to the US Geological Survey (USGS).

"There has been no damage to houses, according to the first reports," a spokesperson for the fire service told AFP.

A 12th century Byzantine monument on a small island near Zante has been severely damaged, local media reported.

The earthquake was also felt on mainland Greece, including in the western town of Pyrgos.

USGS said the quake, which struck at a depth of 16.6 kilometres (10 miles), was preceded by a shake of 5.0 and followed by a series of smaller aftershocks.

It said there was a low likelihood of casualties.

The Athens earthquake observatory said the tremor was felt strongly in Zante, a popular holiday destination with its own international airport.

Greece lies on major fault lines and is regularly hit by earthquakes, but they rarely cause casualties.

However, in July 2017 a magnitude 6.7 earthquake killed two people on the island of Kos in the Aegean sea, causing significant damage.

Zante island experienced a major earthquake in 1953 of 6.4, killing hundreds of victims and injuring thousands, leaving the island in ruins.