Rebels say they have advanced into several central areas in Aleppo
A four-day ceasefire to mark the Muslim holiday of Eid al-Adha has come into effect in Syria, but the army warns it will retaliate against rebel attacks.
It was proposed by UN and Arab League envoy Lakhdar Brahimi, who hopes it will lead to a peace process.
The truce began at 06:00 (04:00 GMT), reported Syrian TV. Previous attempts at ceasefires in Syria have collapsed.
On Thursday rebels advanced into central areas of Aleppo, Syria's largest city and a key battleground.
The US welcomed the ceasefire, and UN Secretary General Ban Ki-moon said he hoped it would lead to political negotiations.
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 that they would do in a normal day," 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 a ceasefire working.
Syria's armed forces said in a statement that "military operations" would cease from 26-29 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 the statement, which was broadcast on state television.
Qassem Saadeddine, a spokesman for the joint command of the Free Syrian Army, the main armed rebel group, said his fighters would back the truce.
"But we will not allow the regime to reinforce its posts," he told Reuters.
The uprising against President Bashar al-Assad's 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.
In Aleppo on Thursday, eyewitnesses and activists said government forces had moved away from military posts in the Christian district of al-Seryan and the neighbouring Kurdish area of Ashrafiyeh.
A rebel spokesman was quoted as saying opposition fighters had also taken the south-western neighbourhoods of Salah al-Din and Suleiman a-Halabi.
