A strong quake with a magnitude of at least 6.4 has struck off Greece on Friday morning (local time), earthquake monitors say, but there were no initial reports of extensive damage or casualties.
Greece's Geodynamic Institute reported Friday's tremor as a magnitude 6.4, 50km south of Zakynthos. The US Geological Survey (USGS) and the European-Mediterranean Seismological Centre, assessed the quake as a magnitude 6.8.
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The tremor was felt in Greece's capital, Athens, and as far away as Italy, Malta, Albania and Libya.
The epicentre of the quake was in the Ionian Sea, 124km southwest of Patras in the Peloponnese. The quake was fairly shallow, only 16.5km below the seabed, which would have amplified the shaking.
The European-Mediterranean Seismological Centre (EMSC) has warned of a possible low-scale tsunami that could last for several hours.
As many as 19.3 million people could have felt the earthquake, according to the USGS, including 54,000 people who may have experienced "strong" to "very strong" shaking.
"Overall, the population in this region resides in structures that are a mix of vulnerable and earthquake resistant construction," the USGS said.
"Some damage is possible and the impact should be relatively localised."
A fire brigade official in Athens said there were no early reports of damage or casualties.
But a spokesman for Greece's civil protection agency told Reuters that three people had been taken to hospital, two of them slightly injured.
Local news websites reported that a 15th century monastery in the Strofades area of the island had been damaged.
There were power cuts and slight damage to the port of Zakynthos, near the epicentre.