The 12 bodies were found inside the wreck of the boat at a depth of 32 meters (105 feet), the coast guard said in a statement. It said the search-and-rescue operation involving two coast guard boats, a helicopter and a team of divers, is continuing.
It was not immediately clear why the boat sank. There was no immediate information on the migrants' nationalities. Although the number of people heading to the Greek islands from the Turkish coast has decreased since the height of the refugee crisis in 2015, hundreds continue to make the crossing each week.
In Spain, the maritime rescue service said it rescued 292 migrants Monday who were attempting to cross the Mediterranean Sea to Europe from North Africa. Spain in recent years has become the biggest entry point for unauthorized migration to Europe.
However, Spanish Interior Ministry statistics show migrant arrivals are down by almost 18% on last year, at just over 8,800.