Young Syrians walk through the rainy Yayladagi refugee camp in southern Turkey. The number of Syrian refugees has hit 1 million, according to the U.N. refu­gee agency. (Bradley Secker/For The Washington Post)

Syrian rebels abducted about 20 U.N. 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 blaming “armed elements” of the Syrian opposition for the abduction and demanding the “unconditional and immediate release” of all the observers.

On a day that the number of Syrian refugees who have fled the fighting officially passed the 1 million mark, the incident highlighted the danger that Syria’s spiraling conflict will spill beyond its borders and draw in not only the country’s neighbors but perhaps also the wider international community. The vast majority of the refugees have sought sanctuary in Lebanon, Jordan, Turkey and Iraq, where they are straining resources and threatening the stability of communities that are often already volatile.

“It seems that lately some people are trying very hard in order to extend the geography of the Syrian conflict,” said Russia’s U.N. envoy, Vitaly I. Churkin, speaking after the Security Council session. He noted that the abduction came two days after Iraqi insurgents killed nine Iraqi guards along with 48 Syrian soldiers who had fled rebel advances into Iraq and were being escorted back to Syria when they were ambushed.

“Somebody is trying very hard in order to blow this crisis up,” Churkin said.

The United Nations’s 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.’s 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.

According to a U.N. statement, about 20 observers on a regular supply mission to the no man’s land between Israel and Syria were detained by a group of around 30 armed fighters near a post that had been damaged in recent fighting and had been evacuated over the weekend. The statement did not give the nationalities of the observers.

The abductions were first publicized in a video 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, clean-shaven and wearing 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 border with Israel,” the man says, referring to Syria’s president. “If they don’t leave within 48 hours we are going to deal with these people as prisoners.”

The mainstream rebel group, the Free Syrian Army, condemned the abductions and said it has no relationship with those holding the observers. Free Syrian Army units in the Golan area have launched a search for the observers and the rebels holding them, said Louay al-Mokdad, the political and media coordinator for the Free Syrian Army.

“This is not the right action to take and we are not responsible,” he said.

A U.N. official who asked not to be named because of the sensitivity of the negotiations said the group appears to be composed primarily of Palestinian fighters loosely affiliated with the armed insurgency. Yarmouk is the name of Syria’s biggest Palestinian refugee camp, in Damascus.

The kidnapping also illuminated an overlooked front in the fierce battle raging for control of Syria. The area where the observers were seized has been a demilitarized zone since 1967, when Israel occupied most of the mountainous region of Syria known as the Golan Heights. Under the terms of the cease-fire that ended the war, troops from neither side are permitted to go there, an arrangement overseen for the past four decades by the U.N. observers.

But as the fighting has spread southward in recent months, Syrians living in Golan villages have joined the rebellion against Assad, drawing Syrian troops into the area to battle them. Several stray shells fired by the Syrian security forces have exploded in Israeli territory, and on a few occasions the Israelis have fired back.

The mandate of the observers had not been adjusted to fit the new circumstances, leaving them dangerously exposed, U.N. officials noted.

“This was something that was not in any way addressed or envisaged,” said Churkin.

Britain to aid rebels

The crisis erupted hours after Britain announced it had secured an exemption to a European arms embargo against Syria that would enable it to supply nonlethal military equipment to the Syrian Opposition Coalition, the umbrella political movement that purports to represent Assad’s opponents.

British Foreign Secretary William Hague told Parliament the supplies would include body armor and armored four-wheel-drive vehicles to enable the rebellion’s civilian leaders to protect themselves.

The $20 million package follows a $60 million offer of nonlethal aid made last week by the United States, part of an increasingly vigorous effort by Western countries and their Arab allies to counter the expanding influence of Islamist radicals over the Syrian rebellion by stepping up aid to the more moderate elements of the opposition.

Fighters with the extremist Jabhat al-Nusra, suspected of ties to al-Qaeda and blacklisted as a terrorist organization by the United States, appear to have played a prominent role alongside Free Syrian Army fighters in the most recent rebel victory in the eastern city of Raqqah.

The city’s last government buildings were overrun Wednesday, according to the Britain-based Syrian Observatory for Human Rights, giving the rebels tenuous control over their first provincial capital.

But in a reminder that the government still controls the skies, warplanes dropped bombs on the city, and residents said thousands of people were fleeing toward the Turkish border for fear of further airstrikes.

Lynch reported from the United Nations. Ahmed Ramadan and Babak Dehghanpisheh in Beirut contributed to this report.