Activists have reported renewed fighting between Syrian rebels and government forces in the Golan Heights, where 44 Fijian peacekeepers were taken by the rebels on Thursday.
They report heavy clashes around the Quneitra crossing, which rebels seized on Wednesday.
The whereabouts of the Fiji peacekeepers is unclear, but it is thought they are being held by an al-Qaeda-affiliated rebel group, al-Nusra Front.
Al-Nusra says they are safe, but its supporters say the group is demanding the release of Abu Mussab al-Suri, a top al Qaeda leader, in exchange for the detained peacekeepers.
Al-Suri, a Syrian national from Aleppo, was captured in Pakistan in 2005 and handed over to Syria, where he is still believed to be detained.
The United Nations says it will spare no effort in seeking the release of the soldiers.
The assurance was made by the Secretary General of the UN, Ban Ki-moon, who says he has demanded their immediate and unconditional release.