Syrian refugees take "selfies" moments after arriving on an overcrowded dinghy at a beach on the Greek island of Kos, after crossing a part of the Aegean sea from Turkey, August 9, 2015. [Photo/Agencies]

IOM estimates people smugglers in Europe have made $10 billion or more since 2000, maybe much more. "They are certainly getting very well paid for their services," Swing said.

Out of a total of 1,005,504 arrivals to Greece, Bulgaria, Italy, Spain, Malta and Cyprus by Dec 21, the vast majority - 816,752 - arrived by sea in Greece, IOM said.

IOM spokesman Joel Millman said it was impossible to forecast how the flow of migrants would evolve in 2016.

"So much is in the balance, the resolution of the Syrian war, and the disposition of the European border protection moves that are being contemplated," he said. "We never thought it would reach this level. We just hope people are treated with dignity."