Syria has re-imposed rules restricting the entry of Iraqi refugees, who have been crossing its border to escape the violence at home.
Syria has been the main destination of Iraqi refugees
It will now require visas, which will be limited to business people, scientists and academics.
Damascus imposed the visa requirement on 10 September, but lifted it a few days later as a goodwill gesture as the Muslim holy month of Ramadan began.
Syria is already home to at least 1.4m of the 4.4m displaced Iraqis.
According to the UN refugee agency, the UNHCR, some 60,000 Iraqis leave their homes every month. About half of the total displaced remain within Iraq's borders.
The flight of Iraqis is the biggest movement of people in the Middle East since the exodus of Palestinians from what became Israel in 1948, says the BBC's Middle East editor Jeremy Bowen.
Strains on system
Until now Syria has been the foremost destination for those who have fled Iraq but a Syrian official told the news agency AFP that the visa requirement had been restored on Monday. The news was confirmed by the UNHCR.
Syria has been struggling to cope with what amounts to a 10% increase in its population, placing strains on the country's housing, health and education systems.
However, this crisis has lacked visibility because there are no refugee camps - the new arrivals are mostly absorbed straight into poor districts of Damascus, our Middle East editor adds.
The UNHCR, which says the move will remove the last safe haven for Iraqis, is urging Damascus to introduce humanitarian visas.
But on Monday the Syrian Foreign Minister, Walid Moallem, accused the international community of failing to discharge its duty towards the refugees in an address to the UN General Assembly.
"The occupying power [the US] spends hundreds of billions of dollars on military operations inside of Iraq but fails to allocate any resources to assist Iraqis who have been forced out of Iraq as a result of occupation and its concomitant repercussions," he said, according to AFP.