About 20 United Nations peacekeeping observers have been captured by armed forces on the Syrian-Israeli border.
According to reports, the group were on a "regular supply mission" on the Golan Heights when they were stopped by the armed men near an observation point.
Some reports suggested the UN observers were from the Philippines, while a video posted on YouTube claimed the gunmen were Syrian rebels.
In the video, which had not been verified, armed men standing in front of a UN-marked vehicle said the staff would not be released until President Assad's forces withdrew from a nearby village.
Their capture comes at a highly sensitive time for Britain's support for the opposition.
Earlier, Foreign Secretary William Hague had announced Britain would supply armoured vehicles and body armour to Syrian opposition forces as it steps up efforts to end a humanitarian crisis of "catastrophic proportions."
He said he had ordered "more active efforts" after securing a relaxation of an EU arms embargo to allow the provision of non-lethal military equipment to protect civilians.