Syria's army and main rebel force said they will cease fire on Friday, in line with an internationally backed truce during a Muslim holiday, but both reserved the right to respond to any aggression.
A peace initiative by UN and Arab League peace envoy Lakhdar Brahimi calls for a truce during the four-day Muslim holiday of Eid al-Adha from Friday marking the end of the hajj pilgrimage.
It was backed this week by the United Nations Security Council, and a spokesman for UN chief Ban Ki-moon said "the world is now watching" to ensure both sides stick by their commitment.
The United States expressed the hope the ceasefire will be respected.
"What we are hoping and expecting is that they will not just talk the talk of ceasefire, but that they will walk the walk, beginning with the regime," US State Department spokeswoman Victoria Nuland said.
The army announced it would adhere to the ceasefire in a statement read on television.
"On the occasion of Eid al-Adha, military operations will cease on Syrian territory as of Friday morning, until Monday the 29th," it said.
But it also said the army would react if "armed terrorist groups continue to fire on civilians and government troops, attack public and private property and use car bombs and improvised explosive devices."
It also warned of a response if rebels reinforce their current positions or receive ammunition, and to any fighters crossing from neighbouring countries.
The Free Syrian Army, chief among many rebel groups battling President Bashar al-Assad's forces, responded positively soon afterwards, saying it too would lay down its weapons as long as regime troops adhere to the ceasefire.
"We will respect the ceasefire from tomorrow morning if the Syrian army does the same," said General Mustafa al-Sheikh of the FSA, which had previously said it doubts Damascus would stand by any commitment.
"But if they fire a single shot, we will respond with 100. So we reserve the right to respond," he said by telephone from Turkey.
He cautioned, however, that he could not speak on behalf of all rebels.
"There is not a unified command for all the factions. We speak on behalf of a big enough number of fighters, but there are other armed factions who follow other commands," Sheikh said.
An April ceasefire announced by Brahimi's predecessor, former UN secretary general Kofi Annan, failed to take hold.
If the latest does hold, it would be the first real breakthrough in halting -- even temporarily -- the 19-month conflict that rights groups say has killed more than 35,000 people.
Eid al-Adha starts with prayers at dawn on Friday, expected at around 5:30 am local time in Syria, or 0230 GMT.
Shortly before Thursday's announcements, there was little sign of the fighting easing, with rebels moving into a strategically important Kurdish district of the main battleground city of Aleppo.
Residents of the Ashrafiyeh neighbourhood said about 200 rebels had entered the area for the first time.
One 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 ceasefire promises.
"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."
Rebels and troops were also battling in the mainly Christian district of Seryan just south of Ashrafiyeh, said residents.
At least 100 people were killed across Syria on Thursday -- 43 civilians, 37 soldiers and 20 rebels, said the Syrian Observatory for Human Rights.
Fierce clashes raged around the Wadi Daif military base between troops and rebels, including fighters from the jihadist Al-Nusra Front, while the army shelled the strategic town of Maaret al-Numan nearby, said the watchdog.
Al-Nusra, which has claimed several deadly suicide bombings, is among the opposition groups that have already refused to accept the ceasefire, labelling it a "trick."
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.
He said he wanted the ceasefire to help create political space for dialogue and for aid to flow in, particularly to Aleppo, Homs in the centre and Idlib in the northwest.
Syria's key ally Iran dubbed the regime's ceasefire declaration a "positive step and worthy of praise."
Foreign Minister Ali Akbar Salehi hoped that "the other sides give an appropriate response to the Syrian government's goodwill and by respecting the ceasefire to pave the way for (the return of) calmness in Syria."
Iran advocates a political solution and dialogue to end the Syrian crisis.
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.
UN rights investigators said they would go after senior regime officials responsible for "crimes against humanity and war crimes."
Meanwhile, Saudi Arabia on Thursday said it had expelled three employees of Syria's consulate in Jeddah for activities "incompatible" with their position, but did not elaborate.