"As we speak more rebels in more than 20 vehicles are approaching and reinforcing the siege around Position 68," a UN source said on condition of anonymity.
Jabhat al-Nusra, the al-Qaeda-inspired group making gains on the Syrian Golan, released a picture of 45 Fijian peacekeepers it has held captive since Thursday. According to the Islamist group, one of the central reasons for detaining the soldiers was "the UN's failure in the face of the massacres committed by the Syrian military against the Syrian people."
Jabhat al-Nusra claimed that the peacekeeping forces on the Golan were covering for Syrian army movements and providing them with assistance. They also noted the decisions taken by the UN and other international organizations against Jabhat al-Nusra – for its involvement in terror.
"In response to the hardships and the UN's involvement against the people of Syria, Jabhat al-Nusra took 45 UN personnel. We stress that those detained with us are safe and in good health. We are providing food and care for them."
No mention was made of any terms for the hostages' release.
Other UN peacekeepers were able to flee from a different encampment that that was also surrounded by rebels of the Nusra Front, al-Qaeda's Syrian affiliate, they said.
The clashes between the militants and the UN forces on Saturday night came after Syrian rebel groups, including the Nusra Front, overran the Quneitra crossing – located on the frontier between Syrian and Israeli controlled parts of the Golan Heights – on Wednesday, seizing 44 Fijian peacekeepers.
The Nusra Front also surrounded the nearby Rwihana and Breiqa encampments, where other UN peacekeepers were holed up.
The firefight began early Saturday at the Rwihana base some 1.5 miles from Quneitra, where 40 Filipino peacekeepers were surrounded by Nusra fighters who were ordering them to surrender, said Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights. The Philippines' Defense Secretary Voltaire Gazmin gave a similar account but did not name the armed group.
Abdurrahman, whose information comes from a network of activists throughout Syria, said he was not aware of any fatalities among the 40 Filipino peacekeepers in the Rwihana encampment as sporadic fighting continued throughout the day. A Philippine military spokesman, Lt. Col. Ramon Zagala, also said there were no casualties.
The UN mission, known as UNDOF, has 1,223 troops from six countries: Fiji, India, Ireland, Nepal, Netherlands and the Philippines.
UN Secretary-General Ban Ki-moon strongly condemned Saturday's attack on UN peacekeepers' positions in the Golan Heights, in a statement released by his spokesman.
"The Secretary General demands the unconditional and immediate release of all the detained United Nations peacekeepers and calls upon all parties to cooperate fully with UNDOF to enable it to operate freely and to ensure full safety and security of its personnel and assets," the statement said.
The 35 Filipino UN peacekeepers at the Breiqa encampment were extracted on Saturday morning, with the assistance of Irish peacekeepers who rushed to the scene, said officials.
The Irish UN peacekeeper battalion, which is tasked with emergency responses, evacuated all the Filipino UN peacekeepers on Saturday morning, said a military official who spoke on condition that his name and country of origin not be revealed, citing army policy.
He said there was no shooting involved, and no injuries. He said that the Irish battalion also evacuated another base on Friday but provided no further details.
Gazmin confirmed that peacekeepers from his country were "extricated." The Philippine military said there were 35 Filipino troops in the encampment.
An Israeli military spokesman confirmed that a number of UN peacekeepers entered Israel. He spoke on condition of anonymity citing military guidelines.
It was not immediately clear which rebel group was holding the Fijian UN peacekeepers, although it was likely to be the Nusra Front, said Syrian activist Abdurrahman.
The Nusra Front has recently seized hostages to exchange for prisoners detained in Syria and Lebanon.
The situation of the peacekeepers, tasked with monitoring a 1974 disengagement accord between Syria and Israel, remains "very, very fluid," the UN secretary-general's spokesman, Stephane Dujarric, told reporters Friday at the UN headquarters in New York.
The UN said in a statement that it had received assurances from credible sources that the Fijian peacekeepers "are safe and in good health."
UN Secretary-General Ban Ki-moon has condemned the detention of the Fijians and called for their immediate release.
Various rebel groups have been engaged in intense fighting with the Syrian military in and near the Golan Heights.
The Associated Press contributed to this report.