Twenty-one people have died after two boats carrying refugees from Turkey to Greece sank, in the latest fatal incidents in the eastern Aegean Sea.

Greece's said 18 people were killed and 138 people were rescued near the island of Kalymnos, while another three died and six were rescued in a separate incident off the island of Rhodes.

The deaths occurred amid a surge of crossings to Greek islands involving refugees from Syria, Afghanistan and other countries ahead of winter and as European governments consider taking tougher measures to try to limit the number of arrivals in Europe.

Eight people were killed early on Thursday after a boat capsized off the island of Lesbos.