About 20 UN observers have been detained by about 30 armed fighters in the Golan Heights on the Syria-Israel border, the UN has confirmed.
It said the observers were monitoring a ceasefire between Syria and Israel.
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 Martyrs of Yarmouk group told the BBC they had taken the observers to stop Syrian troops from shelling them.
The UN mission in the Golan Heights is sending a team to assess the situation and negotiate the observers' release.
Disengagement Observer Force (Undof) were on "a regular supply mission" when they were stopped near an observation post 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 rebels later admitted taking the monitors to try to stop the Syrian army from firing on them and civilians in the areas.
The rebels added that the UN team was their guests.
The video was circulated by the London-based Syrian Observatory for Human Rights (SOHR).
The SOHR is one of the most prominent organisations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified.
Meanwhile, the Free Syrian Army (FSA) - the main rebel fighting force - condemned the seizure of the UN observers.
FSA leader Gen Salim Idriss told the BBC's Newshour programme he would "do everything what I can to liberate them".
UN Secretary General Ban Ki-moon and the UN Security Council also condemned the detention of the observers and demanded their immediate release.
In a separate development, the New York-based pressure group Human Rights Watch said it was investigating whether the same rebels were involved in the executions of seized Syrian government soldiers earlier this month.
UK armour pledge
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 recognised.
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.
Earlier on Wednesday, the UN High Commissioner for Refugees said the number of Syrian refugees who had fled the conflict reached a million.
It said half of the refugees were children - most of them under 11 and often traumatised by their experiences.
The largest numbers of refugees were seeking shelter in Jordan, Lebanon, Turkey, Iraq and Egypt.
And in a separate development, Britain said it would provide armoured vehicles and body armour to opposition forces in Syria "to help save lives".
Foreign Secretary William Hague said London would offer millions of pounds in "non-lethal" equipment, including search and rescue, communications, and disease-prevention materials.
Up to 70,000 people have been killed and a million refugees have fled since the crisis in Syria began two years ago.