BEIRUT -- New video yesterday of 21 UN peacekeepers held captive by Syrian rebels illustrates the sudden vulnerability of a UN force that had patrolled a cease-fire line between Israel and Syria without incident for nearly four decades.

The abduction of the Filipino troops, soft targets in Syria's civil war, also sent a worrisome signal to Israel about the lawlessness it fears along the shared frontier if Syrian President Bashar Assad is ousted.

Opposition fighters seized the peacekeepers Wednesday near the Syrian village of Jamlah, just a mile from the Israeli-controlled Golan Heights, a plateau Israel captured from Syria in 1967. Negotiations were under way yesterday for the release of the men, who said in videos posted online that they were being treated well.

A rebel spokesman seemed to suggest, however, that the hostages were also serving as human shields. If the UN troops are released and leave the area, the regime could kill "as many as 1,000 people," said the spokesman, who spoke via Skype and did not give his name for fear of reprisals.

Fighting has spread across the country, claiming more than 70,000 lives and displacing nearly 4 million of Syria's 22 million people, with 2 million seeking cover in camps and makeshift shelters across the nation.

The humanitarian crisis hit a grim milestone Wednesday: The number of UN-registered refugees topped 1 million, half of them children. The registered refugees are in Turkey, Jordan, Lebanon, Iraq and Egypt. Several hundred thousand more fled the country but haven't signed up with the UN refugee agency.

The peacekeepers' abduction highlights the growing risks to UN staff in Syria. The capture of the 21 peacekeepers will almost certainly lead to a re-examination of security for the UN monitors and their patrols in the field, diplomats said.

The UN mission was set up in 1974, seven years after Israel first captured the Golan and a year after it managed to push back Syrian troops trying to recapture the territory in another regional war.

The peacekeepers' four-vehicle convoy was intercepted Wednesday by rebels from a group calling itself the Martyrs of the Yarmouk Brigades. The convoy was stopped on the outskirts of Jamlah, about a mile from the armistice line.

Rebels said 10 people have died in recent days in regime shelling of Jamlah and nearby villages, where fighting continued yesterday, according to activists.

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