The vessel was carrying around 50 migrants and 21 of them were rescued.
A dozen migrants have been found dead after their boat sank off the Greek island of Paxos, the country's coastguard said.
It sank on Saturday morning after taking on water.
There were believed to be 50 migrants onboard and 21 have been rescued.
This latest incident comes amid a busy week for migrant sea rescues.
On Thursday and Friday, 119 people — including 50 children — were rescued in the Mediterranean Sea, according to the NGO Sea-Watch. There were two rescues in Libyan waters and another one near Malta.
Crisis-hit Libya has witnessed months of fighting between the self-styled Libyan national army led by General Khalifa Haftar and UN-backed Libyan government.