Rebels fighting the Syrian government seized the only border crossing operated by United Nations peacekeeping forces along the Israeli-Syrian cease-fire line in the Golan Heights on Thursday, according to the Israeli military and rebel forces, bringing the Syrian conflict ever closer to Israeli-held territory.

The mayhem was enough to threaten the continuation of the United Nations Disengagement Observer Force, the longtime peacekeeping mission in the sensitive and disputed area. The Quneitra crossing is patrolled by Austrian United Nations peacekeepers, who were ordered to pull back for their own safety. Twice in recent months a rebel group has taken groups of Filipino peacekeepers in the same mission captive, releasing them unharmed after several days.

One senior Israeli government official said that in general there was “skepticism among the senior Israeli leadership as to the utility of international forces or monitors when things get tough.” The Israeli government has long insisted on a doctrine of self-reliance regarding its security. But analysts said that a disintegration of the United Nations peacekeeping force could certainly complicate the situation along the Israeli-Syrian frontier.

Amnon Sofrin, a retired Israeli brigadier-general told reporters in a telephone briefing that the rebels were sending a message to President Bashar al-Assad of Syria that they were prepared to attack the government’s symbols after Syrian government forces and their Hezbollah allies from Lebanon pushed the rebels out of their symbolic stronghold of Qusayr this week. Mr. Sofrin said that if the United Nations peacekeeping force withdrew, the buffer zone on the Golan Heights would become “a no-man’s land” where Israel might have to face the rebels more directly.