GAZIANTEP, Turkey – Syrian rebels abducted around 20 United Nations observers from the Golan Heights on Wednesday and threatened to hold them until the Syrian government withdraws its troops from the area, marking the most serious escalation of the conflict yet along Syria’s southern border with Israel.

In New York, the U.N. Security Council swiftly issued a statement condemning “armed elements” of the Syrian opposition for the abduction and demanded the “unconditional and immediate release” of all the observers.

The United Nations’ top peacekeeper, Herve Ladsous, indicated that negotiations had already begun to secure the freedom of the observers, who serve as part of the U.N. Disengagement Observer Force monitoring the 1967 cease-fire line between Syria and Israel.

“It’s a very serious incident. The situation is ongoing, negotiations are going on, and the matter is mobilizing all of our teams,” Ladsous told reporters after briefing the council.

The abductions were first publicized in a video that was posted on the Facebook page of a rebel group calling itself the Yarmouk Martyrs’ Brigade.

The camera pans to show several white armored vehicles painted with the U.N. logo as a fighter, dressed in a black woolen hat, calls upon “America and the U.N. Security Council” to address the group’s demands.

“We won’t release them unless Bashar al-Assad’s troops withdraw from the village of Jamlah on the borders with Israel,” the man says. “If they don’t leave within 48 hours, we are going to deal with these people as prisoners.”

A U.N. statement said about 20 observers had been stopped and detained by a group of about 30 armed fighters earlier in the day.

The U.N. observers were on a regular supply mission when they were stopped near a post that had been damaged in the recent fighting and had been evacuated over the weekend.

The rebel Free Syrian Army condemned the abductions and said the group holding the observers is not part of its structures.