UN Secretary-General Ban Ki-moon has welcomed a planned ceasefire in Syria for the Eid al-Adha holiday.
He said it was important that Syrian government troops and armed opposition groups adhere to the truce.
Syria's army command announced a ceasefire earlier to mark the Muslim holiday but said it reserved the right to respond to any rebel attack or moves to reinforce President Bashar al-Assad's armed foes.
It said military operations would cease from tomorrow to Monday.
A Free Syrian Army commander gave qualified backing to the truce, proposed by UN-Arab League envoy Lakhdar Brahimi, but demanded Mr Assad free detainees.
An Islamist group said it was not committed to the truce, but may halt operations if the army did.
Mr Ban's spokesman Martin Nesirky said it was in everybody's interest that the fighting stops tomorrow.
Activists say more than 32,000 people have been killed in the 19-month conflict, which began as popular protests and escalated into civil war.
"It's important that all sides will adhere to this. We all understand that there is a lack of trust between parties and therefore we all understand that we cannot be sure what will transpire," Mr Nesirky told reporters.
"We would simply, fervently hope 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," he said. "The world is now watching to see what will happen on Friday."
The fighting pits mainly Sunni Muslim rebels against President Assad, from the Alawite faith which is linked to Shia Islam, and threatens to draw in regional Sunni Muslim and Shia powers and engulf the whole Middle East, Mr Brahimi has warned.
The UN Security Council backed Mr Brahimi's proposed ceasefire on Wednesday, but has been deadlocked over taking any stronger action to try and end the conflict.
The United States and European council members blame Russia, a staunch ally and key arms supplier for Mr Assad's government, and China for the council's inaction on the conflict.
Moscow and Beijing have vetoed three resolutions condemning Mr Assad and reject the idea of sanctioning his government.