Syria's army said it will cease military operations on Friday, in line with an internationally backed truce during a Muslim holiday, but that it reserves the right to respond to rebel aggression.
"Military operations will cease on Syrian territory as of Friday morning, until Monday," the army said in a statement read on state television.
"(The military) reserves the right to respond to continuing attacks on civilians and government forces by armed groups."
A peace initiative by UN and Arab League peace envoy Lakhdar Brahimi calls on both sides to observe a truce during the four-day Muslim holiday of Eid al-Adha that begins Friday.
The rebel Free Syrian Army (FSA) did not immediately respond to the announcement, but it had been wary of any commitment from the regime of President Bashar al-Assad, as was the United States.
If it holds, a ceasefire would mark the first real breakthrough in halting -- even temporarily -- the 19-month conflict that rights groups say has killed more than 35,000 people.
Shortly before the announcement, there were no signs of a slowdown in the fighting, with rebels moving into a strategically important Kurdish neighbourhood in the main battleground city of Aleppo.Residents in Aleppo's Ashrafiyeh district -- a key area in the heights of the city on a route between its central and northern parts -- said about 200 rebels had moved in to the area for the first time.
One resident said the rebels, who arrived on vehicles mounted with heavy machineguns and bearing the markings of the Liwa al-Tawhid main rebel unit, made it clear they were settling in for Eid despite the promises of a ceasefire.
"Snipers have set up in the buildings and 50 armed men, dressed in black and wearing headbands with Islamic slogans, entered a school near me. I heard them tell the residents: 'We are here to spend Eid with you'," he told AFP.
"I am waiting for things to calm down before leaving," he said.
Rebels and troops were also locked in fighting in the mainly Christian district of Seryan just south of Ashrafiyeh, according to residents.
Fighting elsewhere saw rebels take control of a military post in the northeastern province of Raqa, regime forces bombing the Damascus suburb of Harasta and battles in the capital's southern areas of Tadamun and Qadam, the Syrian Observatory for Human Rights said.The watchdog gave an initial toll of 42 people -- 26 civilians, 10 soldiers and six rebels -- killed on Thursday across Syria, including 12 by mortar fire in Aleppo's Ashrafiyeh.
The Free Syrian Army had previously said it would only agree to the temporary truce if regime troops cease fire first and that it doubts Damascus will stand by any commitment.
But other rebels groups have 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".
The United States has also voiced scepticism, with US envoy to the UN Susan Rice saying many would doubt the regime's word "given Assad's record of broken promises".
Still, Brahimi stressed that "if we succeed with this modest initiative, a longer ceasefire can be built" that would allow the launch of a political process.
A ceasefire announced by his predecessor Kofi Annan in April failed to take hold.Brahimi said he wanted the ceasefire to help create political space for dialogue and for aid to flow in, particularly to the cities of Aleppo in the north, Homs in the centre and Idlib in the northwest.
And the UN's refugee agency said it was ready to send emergency aid to thousands of Syrian families in previously inaccessible areas if the ceasefire holds.
"In all, some 550 tonnes of supplies are being made available for distribution to up to 13,000 affected families -- some 65,000 people -- in several previously inaccessible areas," said the UN High Commissioner for Refugees.
Meanwhile, the international community was boosting pressure on the regime, with UN rights investigators saying they would go after high-ranking officials responsible for atrocities.
Former war crimes prosecutor Carla del Ponte, who joined the UN Commission of Inquiry on Syria last month, said that without a doubt "crimes against humanity and war crimes" were being committed in Syria.
Del Ponte said she was focusing on determining "the high-ranking political and military figures (responsible for) these crimes".
The UN investigators also said they had sent a letter to Assad seeking access to the country, after being barred from seeing the conflict first-hand since the commission was created just over a year ago.