The coast guard said Tuesday it had rescued seven people and had located eight bodies, two of which were still trapped inside the overturned vessel. Crews were searching for between three and five more people listed as missing.
It was not immediately clear how the boat overturned, or what the passengers' nationality was.
Europe faces a massive refugee crisis. More than 600,000 people have made their way through Greece so far this year, fleeing war and poverty at home. The vast majority do not stay in the country, heading north through the Balkans to more prosperous European Union countries.
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