A boat with 27 migrants on board sank in the waters between Turkey and the Greek island of Chios on Tuesday.
The Greek coastguard was able to rescue 20 people.
Seven people are still missing, Greek state radio ERT reported, citing the coastguard.
A NATO ship, two helicopters and several coastguard boats, as well as passing ships, are taking part in the rescue operation.
The local media reported that strong winds is hampering the search.