The International Organization for Migration says its estimate of the number of Syrians who have arrived in Greece this year has nearly doubled in just the last four days — to more than 175,000 people.

The Geneva-based intergovernmental agency attributes the jump to both an accelerated influx of Syrians from the Turkish coast to nearby Greek islands in recent days and a lag by overwhelmed Greek authorities in determining the nationalities of those arriving.

The agency said that as of Thursday, a total of 175,375 Syrians have reached Greece this year, up from 88,204 counted in its previous tally on Monday. Overall, it estimates that 432,761 migrants have crossed the Mediterranean this year.

IOM's tally of Afghans, the second largest contingent, rose to more than 50,177 from 32,414 previously.