The UN refugee agency has warned that new Syrian visa restrictions imposed on Monday are preventing Iraqi refugees from escaping violence and persecution.
Syria is struggling to cope with the massive influx of Iraqi refugees
Officials from the UN High Commissioner for Refugees (UNHCR) said the usually busy border crossing between Iraq and Syria had become "virtually empty".
Syria used to allow all Iraqis to stay six months without a visa, but now only business people and academics may.
Damascus says it is struggling to cope with an estimated 1.4m Iraqi refugees.
The crisis is said to be costing Syria around $1bn a year and has placed exceptional strain on the country's housing, health and education systems.
Jordan imposed tighter visa restrictions on Iraqis two years ago.
A spokesman for the UNHCR, Ron Redmond, said the new Syrian visa restrictions had had a "direct and immediate impact on the numbers of Iraqis crossing that border".
"The regulations effectively mean there is no longer a safe place outside for Iraqis fleeing persecution and violence," he told a news conference in Geneva.
"We are increasingly concerned about their fate, as their options for safety are reduced."
Mr Redmond said Iraqis were now required to apply for visas at the Syrian embassy in Baghdad, but that many were unable or unwilling to travel to the capital because of security concerns.
Many Iraqis reportedly rushed to cross over the border a day before the restrictions were imposed in order to beat the deadline.
Mr Redmond said UNHCR officials were seeking clarity from Syria on what other implications the new rules would have.
"We're also discussing with [Syria] our proposal for a humanitarian visa so that people in need of protection can still get it," he said.
"They've got to have somewhere to go," he said. "We're still hoping that some arrangement can be worked out."
The UNHCR estimates that about 2,000 Iraqis are leaving their homes every day to escape violence, persecution and economic uncertainty.