Ten people, including children, have died after four boats sank off Greece in the eastern Aegean Sea on Wednesday.
Hours earlier the Greek coastguard had rescued 242 people from a sinking boat in the area.
Thousands have been arriving on the island of Lesbos every day after making the dangerous journey from Turkey.
Meanwhile, many are still using the perilous route from the Libyan coast – Italian authorities say 1,042 people were rescued in the area on Wednesday.
Seven separate rescue operations were carried out on Wednesday morning, Italian media reports say.
British, Irish, German and Slovenian naval vessels also took part in the rescues off Libya, the AGI agency reported.
The vessels are acting as part of the EU’s EUNavfor operation.
More than 30 people are still missing from one of the sinkings, of a wooden boat, in the waters off Lesbos, Greek officials say.
Images from the rescue on Wednesday showed emergency workers attempting to revive unconscious children once they had been brought to Lesbos.
Seven children are believed to be among those who died, the charity Save the Children said.
According to the charity, 70 children have drowned trying to reach Greece since Alan Kurdi’s tragic death almost two months ago.
An image of Alan lying face down on the beach sparked an international outcry over the human cost of the crisis.
The UN estimates more than 700,000 migrants have crossed to Europe by boat so far this year – many of them from war-ravaged Syria. The approach of winter has so far done little to slow the flow.
Also on Thursday, German Foreign Minister Frank-Walter Steinmeier visited Athens, promising more help for the Greek authorities.
“At a moment when Greece is trying to recover economically, this influx is an extraordinary burden,” he told Greek newspaper Ta Nea.
Later he is expected to meet Greek Prime Minister Alexis Tsipras and Foreign Minister Nikos Kotzias to discuss the crisis as well as the implementation of reforms demanded by creditors in exchange for the country’s latest bailout.
In a separate incident, a body, thought to be that of a missing four-year-old Bosnian boy, was found in a car near Berlin yesterday. The boy went missing a month ago while at a refugee registration centre in Berlin with his family.
Prosecutor Michael von Hagen told reporters on Thursday that the suspect was turned in by his mother after she recognised him in a video released by police. He has since confessed.
Germany is seeing a huge influx of refugees and migrants – around a million this year alone.
