A spokeswoman for Secretary General Ban Ki Moon said in a statement that the observers - reportedly from the Philippines - were on a regular supply mission and were stopped near an observation post which had been damaged during heavy fighting in the nearby village of Jamlah.
Two videos posted online showed a convoy of U.N. vehicles that had been stopped by a group calling itself the Yarmouk Martyrs Brigade. In the first, a young man reads a statement in which he says his group will hold the observers until the Syrian army withdraws from the outskirts of Jamlah.
"If no withdrawal is made within 24 hours we will treat them as prisoners," the rebel spokesman said.
Men wearing the U.N.'s trademark robin's egg blue can be seen sitting in the vehicles.
Rami Abdel Rahman of the watchdog Syrian Observatory for Human Rights said he had been in touch with a leader of the rebel group said the rebels, which number about 30, have not threatened to kill the observers and that for now the U.N. troops are being held and "treated as guests in our village."
The group said it had attacked a nearby army base, killing 16 Syrian soldiers. Another online video purports to show the group executing forces loyal to Syrian President Bashar al-Assad.
Abdel Rahman said the group's member told him they had recently taken control of Jamlah and now the military is mounting a counterattack which will happen unless the U.N. intervenes.
"The members of the Security Council demanded the unconditional and immediate release of all the detained U.N. peacekeepers and called upon all parties to cooperate with UNDOF in good faith to enable it to operate freely and to ensure full security of its personnel," said Russia's U.N. Ambassador Vitaly Churkin, who is currently serving as president of the Security Council.
The lightly-armed observer mission, called UNDOF, has been in the Golan Heights to monitor the buffer zone between Israel and Syria since 1974. An Israeli defense official told ABC News that Israel had offered to help, but was told the U.N. could handle it. The official declined to elaborate on what help would entail.