UN humanitarian coordinator Yacoub El Hillo said the world body hoped to take advantage of the first major truce in five years of conflict to distribute supplies to an extra 154,000 people living in besieged areas over the next five days.
A successful truce would also create a more favourable backdrop for peace talks that collapsed in acrimony in early February as a Russia-backed regime offensive in northern Syria caused tens of thousands to flee.
UN envoy Staffan de Mistura aims to relaunch the talks on March 7 if the ceasefire lasts and more aid is delivered.
The main opposition grouping Sunday described the ceasefire as "positive" but lodged a formal complaint with the United Nations and foreign governments about breaches.
"We have violations here and there, but in general it is a lot better than before and people are comfortable," said Salem al-Meslet, spokesman for the Saudi-backed High Negotiations Committee.
Meslet said the opposition would like to see the truce "last forever" and that it was the "responsibility of the United States to stop any violations".
An HNC letter to UN Secretary General Ban Ki-moon accused the Syrian regime and its allies of committing "24 violations with artillery shelling and five ground operations... in 26 areas held by the moderate opposition".
It said the breaches had killed 29 people and wounded dozens.
The HNC has said it did not receive any maps of areas included in the ceasefire or documents explaining the monitoring mechanism.
Syria's Al-Watan newspaper, which is close to the government, said Sunday that those maps were still being "kept secret".
The ceasefire does not apply to territory held by the Islamic State group and Al-Qaeda affiliate Al-Nusra Front.
The jihadists of IS last week cut the government's sole supply route to territory it holds in and around second city Aleppo.
After several days of deadly clashes, the army succeeded in reopening it on Monday, the Syrian Observatory for Human Rights said.
At least 26 pro-government fighters and 14 IS jihadists were killed in the fighting around the town of Khanasser, the Britain-based monitoring group said on Sunday. Russia, which has waged a five-month bombing campaign to support Assad, accused "moderate" rebels and jihadists of nine ceasefire violations. But "on the whole, the ceasefire regime in Syria is being implemented," Lieutenant General Sergei Kuralenko, head of Moscow's coordination centre in Syria, was quoted as saying by Russian news agencies.
In Damascus, student Mehdi al-Ani spent Sunday at his university's cafe with friends.
"Yesterday, we only heard two or three shells -- but I pretended like I didn't hear anything. The ceasefire will continue, God willing," he said.
In Aleppo, the Observatory reported some rebel rocket fire on government-held neighbourhoods early on Monday but no casualties.
Children strolled to schools in the city without hugging walls for fear of air strikes or rocket fire, an AFP correspondent reported.
"Our teachers used to forbid us from going out to the school yard because of the air strikes but today we went out and played," said Ranim, a 10-year-old pupil at a primary school in the rebel-held neighbourhood of Bustan al-Kasr.
The Observatory reported nine Russian air strikes on a town in the central province of Hama early on Monday but had no immediate word on any casualties.
Washington urged patience from all sides to give the truce a chance to firm up.
"Setbacks are inevitable," a senior US administration official said.
"Even under the best of circumstances, we don't expect the violence to end immediately. In fact, we are certain that there will continue to be fighting, in part because of organisations like ISIL (Islamic State) and Al-Nusra."
There has been no let-up in the US-led air campaign against IS since the truce went into effect.
The coalition said that on Sunday it carried out 12 strikes against the jihadists in Syria, four of them around the town of Tal Abyad on the Turkish border where IS has been attempting to regain territory from US-backed Kurdish forces.
- AFP
