GENEVA (AFP) - Up to 600,000 migrants and refugees are expected to cross from Turkey to Greece and onwards over the next four months, the United Nations said on Thursday (Nov 5), with the flow set to persist through the winter.

The UN refugee agency (UNHCR) released the figures in a planning document that highlights the additional needs of migrants moving toward or within Europe as the weather turns cold.

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

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

In previous years, migrant crossings of the Mediterranean Sea dropped significantly in the winter, as the harsher weather made the hazardous journey even more difficult.

But UNHCR said it does not expect a similar reduction this year, which has already seen more than 752,000 people cross the sea, most of whom have landed in Greece.

"Despite the onset of winter, it is not anticipated that these movements will decrease," it said, warning the continuing crossings could have dire consequences.

Nearly 3,500 people have already lost their lives trying to cross the Mediterranean to Europe so far this year.

"For those continuing to arrive in Europe, progressively harsh wet and cold winter conditions will only exacerbate the already existing hardships, and may result in further loss of life if measures are not taken urgently," UNHCR said.

The agency has asked for US$96.15 million (S$135 million) in additional support for its winter protection plan, with the focus on adapting shelter and reception centres to cope with colder weather.

Including the US$76.5 million already asked for, UNHCR said it would need a total of nearly US$173 million for its winterisation activities in Europe through 2016.