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BRUSSELS (Reuters) - Nearly half a million irregular migrants arrived in Greece in the last three months of 2015, most of whom then moved north through the Balkans, data from EU border agency Frontex showed on Thursday.

Frontex, which collates data on the number of irregular border crossings, recorded 484,000 such incidents on the Eastern Mediterranean route from Turkey to Greece between October and December and 466,000 on the Western Balkan route, notably people re-entering the European Union at the Croatian border from non-EU Serbia.

That took the total number of illegal EU border crossings, not using regular crossing points, to 978,300 in the quarter, a record since Frontex began collating such data in 2007.

Of those arriving in Greece, mostly on islands off the Turkish coast, 46 percent said they were Syrian and 28 percent Afghan, Frontex said.

It recorded a drop in arrivals in Italy from Libya but noted a sharp increase in arrivals in Spain from Morocco, albeit at a low level. There were 2,800 illegal crossings in the fourth quarter on the Western Mediterranean route, it said, a record for that season and double that in the same period of 2014.