The number of migrants and refugees entering Europe so far this year is expected to reach one million in the coming days after the International Organisation for Migration (IOM) revealed 991,000 have arrived so far.

Some 4,300 people arrived on the Greek islands on Wednesday, including 3,000 on Lesbos, meaning that the exodus of people fleeing conflict and poverty in the Middle East and Africa via Turkey is continuing despite winter weather and rougher seas, it said.

"We see that the flows are so strong even now this late in the year that perhaps by Tuesday, even before then, IOM estimates the 1 million person mark will pass," IOM spokesman Joel Millman told reporters.

The numbers of people reaching the continent in Europe's worst migration crisis since World War Two would be at least four times more than for 2014, he said, adding this was "extraordinary" and a record.

"We also want to point out that these very high flows for December in Mediterranean are proving to be what we feared - very lethal," Millman said, speaking on International Migrants Day.

This was particularly the case in the eastern Mediterranean, or Aegean, where the IOM has counted 706 deaths so far this year. The toll includes 422 since Oct. 16, an average of seven a day, he said.

"Obviously this is quite alarming and we are not looking forward to a winter like last year. But from what we can see the flows remain robust and dangerous," Millman said.

The number of people forcibly displaced worldwide is likely to have "far surpassed" a record 60 million this year, mainly driven by the Syrian war and other protracted conflicts, the United Nations said earlier on Friday.