UNITED NATIONS (AP) — Armed men detained four U.N. peacekeepers on patrol on the Golan Heights in a tense area that separates Syria and Israel, the U.N. said Tuesday. A rebel group claimed it was holding the peacekeepers.

It was the second time in two months that unarmed U.N. military observers have been taken captive, and illustrated again the vulnerability of the U.N. peacekeeping mission amid the spillover from the war in Syria.

Kieran Dwyer, spokesman for the U.N. peacekeeping department, said the four peacekeepers, all from the Philippines, were taken by an unidentified armed group near Jamla on Tuesday. “Efforts are underway to secure their release now,” he said.

A spokesman for the rebel group holding the four peacekeepers said in a phone interview with The Associated Press that at least three of the UN staffers are from the Philippines. He said the rebel unit holding them is called the Yarmouk Martyrs Brigades and the group is operating in the Golan Heights.

The man spoke on condition of anonymity because he is outside of Syria and is serving as a mediator on peaceful matters concerning the group.

In a statement that was posted on the group’s Facebook page Tuesday, the Yarmouk Martyrs Brigades said the peacekeepers are not their hostages, but are being kept with them for their own safety.

In early March, the same rebel group held 21 Filipino U.N. peacekeepers for four days before releasing them unharmed after tough negotiations.

The rebel unit said that it suspects that peacekeepers are believed to be shielding Syrian President Bashar Assad’s troops who the rebels said killed civilians during an army sweep of Wadi Raqat, a town in southern Syria.

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

For nearly four decades, the U.N. monitors have helped enforce a stable truce between Israel and Syria.

But in recent months, Syrian mortars overshooting their target have repeatedly hit the Israeli-controlled Golan. In Israel’s most direct involvement so far, Israeli warplanes struck inside Syria in January, according to U.S. officials who said the target was a convoy carrying anti-aircraft weapons bound for Hezbollah, a Lebanese militia allied with Assad and Iran.