ROME (Reuters) - Rescuers found 13 dead migrants on a boat off the coast of Libya with more than 500 others aboard, an Italian coast guard spokesman said on Tuesday, giving no details about how they had died.

An Irish naval ship, mobilized as part of the EU's Triton mission, and one operated by the Doctors Without Borders charity went to the rescue of the boat about 50 miles (80 km) off Libya late on Monday. The bodies were taken onto the Irish ship.

Authorities are yet to decide where to take the survivors, the spokesman said.

Some 150,000 migrants have reached Europe by sea this year, most landing in Italy and Greece, and more than 1,900 have drowned during the crossing, according to the International Organization for Migration.