Human Rights Watch said it was investigating the same rebel brigade that seized 20 United Nations peacekeepers on Wednesday for executing captured Syrian soldiers earlier this week.
Peter Bouckaert of the human rights monitoring group told Reuters it was investigating the involvement of the "Martyrs of Yarmouk" brigade in a videotaped execution of detained Syrian soldiers posted on the Internet on Tuesday.
The UN says about 20 peacekeepers in the force charged with monitoring the cease-fire between Israel and Syrian troops on the Golan Heights have been detained by approximately 30 armed fighters.
UN deputy spokesman Eduardo del Buey says the UN observers were on a regular supply mission Wednesday when they were stopped near an observation post which sustained damage and was evacuated last weekend following heavy combat.
He said the UN peacekeeping mission has dispatched a team to assess the situation and attempt a resolution.