Syria's army and main rebel force confirmed on Thursday they will cease fire on Friday, AFP reported, in line with an internationally backed truce during a Muslim holiday.
However, the report said, both sides 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 starting on Friday.
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," U.S. State Department spokeswoman Victoria Nuland said, according to AFP.
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.
The statement also warned 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 neighboring 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 told AFP by telephone from Turkey.
Al-Sheikh 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," he 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.
Shortly before Thursday's announcements, reported AFP, 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 neighborhood said about 200 rebels had entered the area for the first time.
Rebels and troops were also battling in the mainly Christian district of Seryan just south of Ashrafiyeh, residents said.
At least 100 people were killed across Syria on Thursday -- 43 civilians, 37 soldiers and 20 rebels, said the Syrian Observatory for Human Rights.
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 center 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."
Salehi previously said that Iran had given a proposal to find an exit strategy for the Syrian crisis, but did not go into details about the proposals.
On Wednesday, the United States’ Ambassador to the UN, Susan Rice, said she doubts the willingness of the Syrian regime to follow through on the ceasefire.
"The atrocities that the regime is carrying out in Syria threaten to compromise the security of the entire region," Rice said. She stressed, however, that the United States supports Brahimi’s efforts to bring an end to the crisis in Syria.