The UN has confirmed that about 20 observers have been detained by about 30 armed fighters in the Golan Heights on the Syria-Israel border.
A video posted earlier on the internet showed men claiming to be Syrian rebels standing next to vehicles with the letters “UN” written on them.
The UN said the observers were monitoring a ceasefire between Syria and Israel.
A team is being dispatched from the UN mission to assess the situation.
UN deputy spokesman Eduardo del Buey said the observers were on “a regular supply mission” when they were stopped near Observation Post 58 by the armed men.
He said the post had sustained damage and was evacuated over the past weekend following “heavy combat in close proximity”.
The spokesman did not provide any further details.
Some reports suggest the UN observers were from the Philippines.
In the video published on the internet, the gunmen identified themselves as the “Martyrs of Yarmouk”.
They are heard saying that the UN personnel would not be released until forces loyal to Syrian President Bashar al-Assad withdrew from the village of Jamla in the area.
The video was circulated by the London-based Syrian Observatory for Human Rights (SOHR).
The SOHR is one of the most prominent organizations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified.
The UN has had its monitors in the area since the 1974 ceasefire between Israel and Syria.
Israel has occupied the Golan Heights since 1967 and later annexed the territory – in a move that is not internationally recognized.
Recently there has been fighting in the eastern foothills of the Golan Heights between Syrian President Bashar al-Assad’s forces and rebels.
Israel has said its policy is not to get involved in the Syrian conflict. However, in recent months it has retaliated when there has been Syrian fire into Israeli-controlled areas.
Israel has also reinforced a fence that runs along the armistice line, and officials say Syrian refugees will not be allowed into Israel en masse.