Four boats carrying refugees from Turkey to Greece have sunk, leaving at least 11 people dead and more than 40 others missing, Greek authorities said on Thursday.

Greece's Coast Guard retrieved 11 bodies, including eight children, and a rescue operation was underway in the Aegean Sea to search for the missing, the authorities said, Xinhua news agency reported.

The sinkings occurred from Wednesday to early Thursday off the Greek islands of Lesvos, Samos and Agathonissi.

Many infants and toddlers were washed up unconscious and medical staff and volunteers attempted to revive the children, with some suffering from hypothermia.

Tragic images show the babies and toddlers wrapped in thermal blankets and surrounded by medics who are seen rubbing their little bodies to warm them. [See photos below]

This is the latest tragedy in the ongoing refugee crisis that has seen thousands of people, including children, dying at sea while attempting to reach safety in Europe from war-torn countries such as Syria and Afghanistan.

From Tuesday to Thursday, the Greek Coast Guard rescued almost 1,000 people attempting to cross over from the Turkish shores in wooden vessels. Adverse weather conditions did not curb the influx of refugees from war-torn Syria.

Latest data from the Greek shipping ministry, the UN High Commission for Refugees and the International Office for Migration show that since January 2015, about 500,000 refugees and migrants have reached Greece to seek a better and secure life in central and northern European countries.

More than 3,000 people have died this year while crossing the Mediterranean Sea.

Greek Shipping Minister Theodoros Dritsas reiterated in a statement Athens' plea to the European Union and the neighbouring countries to step up coordination to avoid such tragedies at sea.

Among the most shocking incidents in the refugee crisis was the photo of three-year-old Aylan Kurdi who was washed up ashore Turkey's beach after he and his mother and sibling drowned while trying to reach Greece.