BEIRUT/GENEVA - Syria's army command announced a ceasefire on Thursday to mark the Muslim holiday of Eid al Adha but said it reserved the right to respond to any rebel attack or moves to reinforce President Bashar Assad's armed foes.A Free Syrian Army commander gave qualified backing to the truce, proposed by UN-Arab League envoy Lakhdar Brahimi, but demanded Assad free detainees. An Islamist group said it was not committed to the truce but may halt operations if the army did.UN aid agencies see window of opportunityUN aid agencies have geared up to take advantage of any window of opportunity provided by a ceasefire to go to areas that have been difficult to reach due to fighting, a UN official in Geneva said."UN agencies have been preparing rapidly to scale up especially in areas that have been difficult to reach due to active conflict and which may become accessible as a result of these developments," he told Reuters.The UN refugee agency UNHCR said that it had prepared emergency kits for distribution for up to 13,000 families - an estimated 65,000 people - in previously inaccessible areas including Homs and the northeastern city of Hassaka."We and our partners want to be in a position to move quickly if security allows over the next few days," UNHCR Syria Representative Tarik Kurdi in Damascus said in a statement.The UN World Food Program has identified 90,000 people in 21 hotspots from Aleppo to Homs and Latakia in need food parcels and will try to reach them through local agencies, the UN official said.Brahimi proposed the temporary truce to stem, however briefly, the bloodshed in a conflict which erupted as popular protests in March last year and has escalated into a civil war which activists say has killed more than 32,000 people.The fighting pits mainly Sunni Muslim rebels against Assad, from the Alawite faith which is linked to Shi'ite Islam, and threatens to draw in regional Sunni Muslim and Shi'ite powers and engulf the whole Middle East, Brahimi has warned."On the occasion of the blessed Eid al-Adha, the general command of the army and armed forces announces a halt to military operations on the territory of the Syrian Arab Republic, from Friday morning...until Monday," an army statement read on state television said.It reserved the right to respond if "the armed terrorist groups open fire on civilians and government forces, attack public and private properties, or use car bombs and explosives."It would also respond to any reinforcement or re-supplying of rebel units, or smuggling of fighters from neighboring countries "in violation of their international commitments to combat terrorism."Qassem Saadeddine, head of the military council in Homs province and spokesman for the FSA joint command, said his fighters were committed to the truce. "But we not allow the regime to reinforce its posts. We demand the release of the detainees, the regime should release them by tomorrow morning," he said.Abu Moaz, spokesman for the Islamist group Ansar al-Islam, which has several brigades fighting in the capital and Damascus province, doubted Assad's forces would observe the truce, though it might suspend operations if they did."We do not care about this truce. We are cautious. If the tanks are still there and the checkpoints are still there then what is the truce?" he said.UN Secretary-General Ban Ki-moon welcomed the announcement and urged the Syrian government and opposition groups adhere to the truce."We would simply fervently hope that the guns do fall silent, that there is a suspension in the violence so that humanitarian workers can help those who are most in need," Ban's spokesman Martin Nesirky told reporters.Brahimi's predecessor, former UN chief Kofi Annan, declared a ceasefire in Syria on April 12, but it soon became a dead letter, along with the rest of his six-point peace plan.Violence has intensified since then, with daily death tolls compiled by opposition monitoring groups often exceeding 200.