Four Filipino UN peacekeepers have been abducted by armed men while patrolling in the demilitarised area between Syria and the Israeli-occupied Golan Heights.
UN spokesmen said efforts were "under way to secure their release now".
A Syrian rebel group, the Martyrs of Yarmouk, published a photo purportedly showing the four men and said they were being held for their own safety.
It said there had been clashes and heavy shelling in the Yarmouk Valley, in the south of the separation zone.
In March, the Martyrs of Yarmouk detained 21 UN observers from the Philippines, before releasing them three days later.
'Safe and sound'
The Golan Heights, a rocky plateau in south-western Syria, has a political and strategic significance that belies its size.
Israel seized the region from Syria in the closing stages of the 1967 Six-Day War, and thwarted a Syrian attempt to retake it in 1973.
Both countries signed an armistice in 1974, after which the UN Disengagement Observer Force (UNDOF) was put in place to monitor the demilitarised zone. It has 917 lightly-armed military personnel from Austria, India, the Philippines, Morocco and Moldova.
UN spokeswoman Josephine Guerrero said the Filipino peacekeepers were abducted on Tuesday while patrolling near Position 86 inside the demilitarised zone, about 4km (2.5 miles) from the Jordanian border.
A spokesman for UN Secretary-General Ban Ki-moon said he "strongly condemns the detention of four UNDOF peacekeepers by armed elements" and "calls for their immediate release".
"The secretary-general calls on all parties to respect UNDOF's freedom of movement and safety and security," Martin Nesirky added.
In a post on its Facebook page, the Martyrs of Yarmouk Brigade insisted the peacekeepers were not hostages. The heavy presence of Syrian government forces posed a danger to the peacekeepers, as well as "criminal elements" in the area, it added.
A member of the Martyrs of Yarmouk, Abu Iyas al-Hourani, later told the Reuters news agency via Skype: "The brigade does not want this issue blown out of proportion like the last incident."
"[The peacekeepers] are safe and sound, and will be handed over as soon as possible."
UN officials told the AFP news agency that there had been increased military activity by Syrian government and rebel forces in the demilitarised zone, and the UK-based Syrian Observatory for Human Rights confirmed there had been recent heavy fighting.