Syrian activists have reported a series of clashes in violation of a ceasefire to mark a four-day Islamic holiday.
Violence appeared to have died down as the ceasefire period began on Friday morning, before reports of fighting in several cities.
Both rebels and the army had said they would only observe the truce if the other side held their fire.
The truce was proposed by UN and Arab League envoy Lakhdar Brahimi, who hopes it will lead to a peace process.
Previous attempts at ceasefires in Syria have collapsed.
Some rebel commanders said they wanted to observe the truce, but that they had little hope that the ceasefire would hold on the ground.
Syrian state TV reported that the ceasefire had begun at 06:00 (04:00 GMT) on the first day of the Muslim holiday of Eid al-Adha. It showed footage of President Assad attending a mosque for morning prayers.
But the Syrian Observatory for Human Rights, an activist network, said clashes had begun at about 10:30 at a military base near Maaret al-Numan town, near the main road between Damascus and Aleppo.
In clashes in Harasta, a Damascus suburb, activists said at least three people had been killed by tank and sniper fire.
The Local Coordination Committees, another activist group, also reported deaths in the cities of Homs and Idlib.
The BBC’s James Reynolds, on the Turkish-Syrian border, heard repeated explosions and automatic weapons fire coming from the Syrian town of Harin, and saw smoke rising from buildings.
There were anti-government protests in several cities, activists said.
The UN refugee agency said it was standing by to send emergency supplies to thousands of families in previously inaccessible areas if the ceasefire took hold.
The International Committee for the Red Cross said if the ceasefire held it would allow much needed relief to reach the Syrian people.
“Any truce would not only allow some of the humanitarian help to reach them, but it would also allow all the people who’ve been holed up in bunkers and at home, and also displaced in camps to rest a bit and to do normal things,” ICRC spokesman Alexis Heeb, told the BBC’s Newsday programme.
‘Backing the truce’
Mr Brahimi has travelled across the Middle East over the past two weeks to promote his plan, and on Wednesday won the support of the UN Security Council. He also said most opposition groups would back the truce, though some rebels expressed scepticism about the chances of it working.
Syria’s armed forces said “military operations” would cease from 26 to 29 October.
“Syrian armed forces will, however, reserve the right to reply to terrorists attacks, attempts of armed groups to reinforce or resupply, or attempts to infiltrate from neighbouring countries,” added a statement broadcast on state TV.
Col Ahmad Hijazi, an officer describing himself as the chief of staff of the Free Syrian Army – the largest armed opposition group – said rebels would not agree to a ceasefire.
“The regime is used to treachery and scheming,” he told the BBC. “It is not to be trusted.”
Rebel spokesman Brig Methqal Husani al-Btaish al-Neemeh said fighters would only observe the ceasefire if the government freed all prisoners, ended aerial bombardments and the siege of Homs, and did not use the truce to resupply.
The US welcomed the ceasefire, and UN Secretary General Ban Ki-moon said he hoped it would lead to political negotiations.
The uprising against President Bashar al-Assad’ government started in March 2011.
Activists say more than 35,000 people have been killed since then, while the UN estimates that at least 20,000 have died.