A United Nations official says the Islamic militant Hamas has prevented a senior employee of the international organization from leaving the Gaza Strip.

The official said Mahmoud Daher, the Palestinian head of the World Health Organization's Gaza office, was barred from leaving Thursday. The official spoke on condition of anonymity because he was not authorized to discuss the incident.

The official said Hamas announced two weeks ago that international organizations would need a Hamas-issued exit permit when leaving the territory into Israel. The organization had previously been exempt from acquiring the permit. The U.N. asked for a month to study the request.

The move appears to be an attempt by Hamas to exert its influence on the U.N., which provides assistance to hundreds of thousands of impoverished Gazans.