The boats sank amid stormy weather as authorities launched a dramatic rescue.
At least 22 people have died and dozens were still missing after two boats carrying migrants sank in Greek waters on Wednesday.
The vessels, which may have travelled from neighbouring Turkey, were hit by stormy weather in the Mediterranean Sea.
Authorities said at least 15 women and one young man had died near the island of Lesbos. A second man was later found dead by divers from the European Union’s Frontex border agency.
Rescuers pulled ten people from the stormy waters but 12 migrants were still missing early on Thursday, Greece's coastguard said.
In a separate incident on Wednesday, at least 11 people were believed to be missing after another boat hit rocks near the island of Kythira -- several hundred kilometres west of Lesbos.
“We could see the boat smashing against the rocks and people climbing up those rocks to try and save themselves,” Martha Stathaki, a local resident in Kythira told AP news agency.
It is believed the boat that sank near Kythira was carrying around 100 people. Rescuers said early on Thursday they had rescued 80 from the waters, while four people had died.
Fire crews lowered ropes to help migrants climb up cliffs on the seafront, with winds in the area reaching 70 kph.
Local officials said a school in the area would be opened to provide shelter for those saved.
Navy divers were also expected to arrive Thursday to help with the search and rescue operation.
Most migrants reaching Greece travel from neighbouring Turkey. People smugglers have changed routes in recent months to try and avoid heavily patrolled waters around Greek islands.
Greek Prime Minister Kyriakos Mitsotakis expressed his "deep sorrow for the tragic loss of life" and praised rescuers’ "heroic" efforts.
“This is a time to really cooperate much more substantially in order to avoid these types of incidents occurring in the future and to completely eradicate the smugglers who prey upon innocent people," Mitsotakis added.
Since January 2022, 64 people have died trying to cross into Europe from Turkey, compared to 111 for the whole of 2021, according to data from the International Organization for Migration (IOM).
Pope Francis visited migrants at a refugee camp on the Greek island of Lesbos last December and labelled them a "shipwreck of civilisation".
Francis blasted what he said was the indifference and self-interest shown by Europe "that condemns to death those on the fringes."
"Please, let us stop this shipwreck of civilisation!" Francis said at the Mavrovouni camp.
Lesbos lies east of the Greek mainland and is around 20km from Turkey's coastline.
"I ask every man and woman, all of us, to overcome the paralysis of fear, the indifference that kills, the cynical disregard that nonchalantly condemns to death those on the fringes," Francis added.
"Let us stop ignoring reality, stop constantly shifting responsibility, stop passing off the issue of migration to others as if it mattered to no one and was only a pointless burden to be shouldered by somebody else."