Four UN peackeepers are being held in the ceasefire line between Syria and the Israeli-occupied Golan Heights, where neither Israeli nor Syrian forces can operate.
Josephine Guerrero, spokeswoman for the United Nations Disengagement Observer Force (UNDOF), said on Tuesday the four were "detained today by an unidentified armed group while they were patrolling" near Al Jamlah in the so-called Area of Limitation.
"The four are from the Philippine battalion. Efforts are under way to secure their release," she said.
Ban Ki-moon, the UN secretary-general, strongly condemned the abductions and called for the peacekeepers' immediate release.
"The secretary-general calls on all parties to respect UNDOF's freedom of movement and safety and security," Martin Nesirky, Ban's spokesman, said.
Reuters reported that a Syrian rebel group had claimed responsibility for the abductions. The news agency quoted the Yarmouk Martyrs Brigades as saying that it was holding four Filipino peacekeepers following clashes in the area that had put them in danger.
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Al Jazeera's Rula Amin, reporting from Beirut in neighbouring Lebanon, said rebel brigades based in the area where the abductions occurred maintain that "[the peacekeepers] are not hostages".
"They [rebels] did not kidnap the UN peacekeepers. They simply took [the peacekeepers] because they want to save their lives," she said.
"Because there is a lot of heavy clashes in the area between government forces and the rebels, they say they are trying to protect the UN observers."
She said "promises" have been made "that the men will be released very soon".
Al Jazeera's James Bays, reporting from the UN headquarters in New York, said the soldiers were a part of UNDOF, which has been in the Golan Heights since the end of the Arab-Israeli war in 1973.
Tuesday's incident follows a similar situation in March involving peacekeepers from the Philippines, and UN diplomats are worried about the continuation of the peacekeeping operation at a time of extreme tension in the region, he said.