European Parliament president Martin Schulz (centre) and Greek Prime Minister Alexis Tsipras  greet a refugee child during their visit to the Moria refugee camp on the Greek island of Lesbos.

AFP/Athens

The cold Aegean Sea claimed the lives of two more migrant children early Thursday, sparking protests in Greece as the UN warned another 600,000 people could arrive by February.

The Greek coastguard said the children died off the island of Kos hours before Prime Minister Alexis Tsipras and European Parliament chief Martin Schulz began a visit to neighbouring Lesbos, another migration flashpoint.

The body of a little girl has been recovered, but the search for the second victim, a six-year-old boy, is still under way.

The boy's father, who was among 14 people rescued from the stricken vessel, had told rescuers that he had been unable to save his son from drowning and had had to let his body go in order to help other passengers.

"A crime is committed in the Aegean (Sea) which must be stopped," Tsipras said after visiting one of the beaches on Lesbos where thousands of migrants and refugees, mainly from war-torn Syria, have been landing this year.

"The situation is dramatic. Much more needs to be done to start managing the situation," said Schulz.

Europe is grappling with its biggest influx of migrants and refugees since World War II, many fleeing conflict and misery in Syria.

More than 750,000 people have made the perilous journey across the Mediterranean this year so far and some 3,400 people have died or gone missing trying to reach Europe, according to UN figures.

The UN refugee agency on Thursday warned that despite deteriorating weather that further raises the stakes for desperate migrants, up to 600,000 additional people are expected to cross from Turkey to Greece over the next four months.

"Between November 2015 and February 2016, UNHCR anticipates that there could be an average of 5,000 arrivals per day from Turkey," the agency said.

'Three days hungry' 

This results in a "up to a total of 600,000 arrivals in Croatia, Greece, Serbia, Slovenia and the former Yugoslav Republic of Macedonia."

However, the expected arrival of some 3mn refugees by 2017 could have a "small but positive impact" on economic growth in the bloc, the European Commission said on Thursday.

Tsipras and Schulz arrived on Lesbos amid a four-day strike by sailors that has prevented thousands of migrants from leaving the island.

The prime minister said over 10,000 people had been stranded at the island's harbour, awaiting ships to Athens' port of Piraeus.

"We are three days hungry. I have three children. My children are sick," a migrant told Tsipras in one of the island's registration centres.

As Tsipras and Schulz landed on the island, a group of protesters entered the Lesbos town hall and unfurled a banner that read "The Aegean is full of migrant corpses, Europeans murderers of peoples".

Other protesters, wearing orange life vests like those worn by thousands in the risky exodus towards Europe, gathered at the so-called hotspot at Moria where new arrivals are registered, and which Tsipras and Schulz also visited.