BEIRUT - Syrian rebels say they have seized a convoy of United Nations observers near the Golan Heights, according to videos posted on YouTube on Wednesday by a violence monitoring group.A young man saying he was from the "Martyrs of Yarmouk" brigade said the convoy would not be released until forces loyal to President Bashar Assad withdrew from the village of Jamla, a mile east of the ceasefire line with the Israeli Golan.Surrounded by several rebel fighters with assault rifles, the man stood in front of a two white armored vehicles and a truck with "UN" written on them. At least five people seen sitting in the vehicles were wearing United Nations light blue helmets and bullet-proof vests."The command of the Martyrs of Yarmouk announced that it is holding forces of the United Nations Disengagement Observer Force until the withdrawal of forces of the regime of (President) Bashar Assad from the outskirts of the village of Jamla," said the man, wearing civilian clothes."If no withdrawal is made within 24 hours we will treat them as prisoners," he said, accusing them of collaborating with Assad's forces to push the rebels out of Jamla. UNDOF supervises a 1974 ceasefire agreement between Syria and Israel.The United Nations said about 20 peacekeepers had been detained by around 30 armed fighters in the Golan Heights on the border between Syria and Israel on Wednesday."The UN observers were on a regular supply mission and were stopped near Observation Post 58, which had sustained damage and was evacuated this past weekend following heavy combat in close proximity, at Al Jamlah," the United Nations said in a statement issued in New York.It did not say the nationality of the observers but the Syrian Observatory for Human Rights monitoring group which is in contact with the rebel brigade said they were Filipino.The United Nations said it has sent a team to resolve the situation, while the 15-member Security Council condemned the incident and demanded the immediate release of the peacekeepers.UN peacekeeping chief Herve Ladsous told the council in an emergency briefing that the fighters who had detained the peacekeepers were "associated with Syrian armed opposition group elements," said Russian UN Ambassador Vitaly Churkin, who is president of the council in March.He added that the captors have made "some demands associated with the actual situation on the ground in Syria." He gave no further detail.Syria's two-year civil war, which has killed more than 70,000 people, has been spilling over into the Golan Heights area.Israel captured the Golan Heights from Syria in a 1967 Six-Day War. Syrian troops are not allowed in the area of separation under a 1973 ceasefire formalized in 1974. Israel and Syria are still technically at war. The area is patrolled by UN peacekeepers.Israel warned the UN Security Council on Monday that it could not be expected to "stand idle" as Syria's civil war spills over its border, while Russia's Churkin accused armed groups of undermining security between the states by fighting in the Golan Heights.