ATHENS: A powerful 6.8-magnitude undersea earthquake struck early Friday off the Ionian Sea island of Zante, causing structural damage but no injuries, officials said.

“We have no reports of injuries, or of someone being trapped under rubble ... We advise people to remain calm,” Zante mayor Pavlos Kolokotsas told Real FM radio.

“Zakynthos has anti-seismic protection ... There were only cracks inside homes and minor rockslides,” said Kolokotsas, using the island’s Greek name.

Electricity in the main town, also called Zante, was temporarily knocked out while officials reported damage to the road network caused by landslides.

The island dock was also damaged, but marine access to the island has not been interrupted.

Many residents spent the early morning outside their homes as a precaution. Island schools would remain shut on Friday.

“We ran out of the house ... The whole village was out on the square,” said Angelike Glava, an elderly local.

The earthquake hit off the southern part of the island, in the Ionian Sea at 1.50am (2250 GMT Thursday), according to the US Geological Survey (USGS).

The Greek national observatory in Athens said the earthquake had a depth of just 10 kilometres.

“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 tremor was followed by over a dozen aftershocks, the national observatory in Athens said.

The earthquake was also felt on mainland Greece, southern Italy and Malta.