Rebels holding 21 UN peacekeepers near the Golan Heights in southern Syria say government forces must stop their bombardment and leave the area before their "guests" can be freed.
A rebel activist said the peacekeepers will be “passed to safe hands when possible - because the area is surrounded and the Assad regime is bombarding it".
Several videos were released of the peacekeepers in which they said they were being well-treated.
However, 24 hours after their capture in the southern village of Jamla there was no indication when they might leave.
The capture of the UN convoy 1.5km from Israeli-held territory was another sign that Syria's conflict, nearing its second anniversary, could spill over to neighbouring countries.
Israel says it will not "stand idle" if violence spreads to the Golan, which it captured in the 1967 Middle East war.
But a senior Defence Ministry official voiced confidence that the United Nations could secure the peacekeepers' release, signalling that Israel would not intervene.
The United Nations says around 70,000 people have been killed in the uprising which erupted in March 2011 with mainly peaceful protests against Assad and has spiralled into an increasingly sectarian conflict.
At least four videos downloaded on the internet showed groups of between three and six peacekeepers, saying they stopped in Jamla for their own safety during heavy bombardment - comments which contrasted with a statement from the United Nations which said they were detained by 30 armed rebels.
The Philippine government condemned the capture of the peacekeepers - three officers and 18 enlisted men - which it called a "gross violation of international law".
President Benigno Aquino told reporters the peacekeepers were being well treated and that the UN was in touch with the rebels to ensure their safety.
Peacekeepers of the UN Disengagement Observer Force (UNDOF) mission have been monitoring a ceasefire line between Syria and the Israeli-occupied Golan Heights for four decades.
The UN Security Council and Secretary-General Ban Ki-moon condemned the seizure of the observers and demanded their immediate release.