Over half a million refugees and migrants have arrived by sea in Greece this year and the rate of arrivals is rising with over 8,000 coming on Monday alone, in a rush to beat the onset of freezing winter, the United Nations said on Tuesday. U.N. refugee agency spokeswoman Melissa Fleming said 27,500 refugees and migrants remained in transit on Greek islands near Turkey from which hundreds of thousands have come. Without this essential element, the relocation programme agreed by Europe in September is in serious peril and may fail." Migrants continue to stream north through the Balkans from Greece but Hungary sealed its border with Croatia on Friday and Slovenia imposed daily limits on migrants entering from Croatia, leaving thousands stuck on cold, rain-sodden frontiers.