Witnesses say Syrian troops have opened fire on a district in Damascus, hours after the country's army agreed to a ceasefire.
The military and the main rebel force agreed to halt military operations from Friday for the Muslim holiday weekend.
But troops stationed on a mountain close to the capital Damascus are said to have shelled Hajar al Aswad, a neighbourhood which is home to many refugees.
In a statement read on state television after the ceasefire was agreed, the Syrian army said it would still respond to gunfire or roadside bombs and keep rebels from bolstering their positions or getting supplies.
"Military operations will cease on Syrian territory as of Friday morning until Monday," the statement said.
"(The military) reserves the right to respond to continuing attacks on civilians and government forces by armed groups."
A Free Syrian Army commander said rebels would also retaliate if they were attacked during Eid al Adha. They also demanded the release of detainees by Friday.
General Mustafa al Sheikh said: "We will respect the ceasefire from Friday morning if the Syrian army does the same. But if they fire a single shot, we will respond with 100. So we reserve the right to respond."
However, the spokesman of Islamist Ansar al Islam, Abu Moaz, said his fighters will not commit to the ceasefire brokered by UN-Arab mediator Lakhdar Brahimi.
Mr Brahimi proposed a four-day ceasefire for the Muslim holiday, saying it could lead to a longer truce and political negotiations between the sides.
Rebel forces have dismissed the idea, saying they don't trust the regime of President Bashar Assad to stick to its promises.
Abu Moaz also said the fighters doubt the government will honour the truce.
If followed through, a ceasefire would mark the first real halt in the 19-month conflict that human rights groups say has killed more than 35,000 people.
Other rebel groups have also refused to accept the proposal, with the radical Islamic Al Nusra Front saying it will not lay down its weapons and denouncing the truce as a "trick".
Fighting continued to rage on the eve of the holiday, with rebel forces clashing with regime troops as they moved into new areas of the second city of Aleppo and battles also continuing elsewhere.