It is unclear whether Syria will enforce the rules for the Iraqis already in the country. United Nations officials believe Syria is likely to continue its practice of not deporting citizens of other Arab countries whose immigration status is illegal.

Syria announced the new rules this summer and said they would take effect on Sept. 1. But it postponed their implementation and continued to accept refugees until Oct. 1. Under the old visa rules, Iraqis entered Syria without restriction and were allowed to remain for three months. Damascus has avoided any announcement about the policy since it took effect, leaving refugees and United Nations officials in a haze of uncertainty.

Under the new rules, Iraqis must apply for visas at the Syrian Embassy in Baghdad. Only academics, merchants with commercial interests requiring travel to Syria, and taxi and truck drivers qualify for visas.

The immediate effect has been to cut the flood of refugees to a trickle, no more than a hundred people a day, according to the United Nations. Over the long term, it means that Iraqi refugees who overstay their three-month visas to Syria may have to make the dangerous trip back to Baghdad and apply to return under the new visa requirement, which disqualifies all but a handful.

Syrian officials have said they were responding to a longstanding request from the Iraqi government to close their border, according to officials who spoke on condition of anonymity, following government policy. They said the Iraqi prime minister, Nuri Kamal al-Maliki, told Syrian leaders on a state visit in August that the constant flow of refugees undermined the Iraqi government’s effort to bring greater security to the country.