Aid workers have made the first delivery of desperately needed assistance since the start of Syria's fragile ceasefire, as an international task force met to try to bolster the truce.
The task force co-chaired by Moscow and Washington held talks in Geneva to evaluate allegations of a range of breaches, said the United Nations, which mediated the three-day-old ceasefire.
At the weekend, key regime backer Russia traded accusations with the main opposition grouping, the High Negotiations Committee (HNC), over truce violations.
On Monday, Russia said seven violations were recorded in the past 24 hours.
However, "the ceasefire between government troops and opposition forces is on the whole being observed", said General Sergei Kuralenko, tasked with monitoring the truce for Moscow, quoted in a defence ministry statement.
The HNC reported two alleged violations by regime forces in Daraa, southern Syria, and two by Russian warplanes in the central province of Hama.
The White House said it was not surprised by reports the ceasefire had been breached, but indicated it was too early to call it a failure.
"We did anticipate that there would be reports of violations and that we would encounter some potholes on the road to implementing this successfully," White House press secretary Josh Earnest said.
UN chief Ban Ki-moon said there had been some incidents but the ceasefire was generally holding.
And in an encouraging sign, aid workers began the first aid delivery since the deal came into effect, the Syrian Arab Red Crescent said.
Twenty trucks carrying blankets and hygiene supplies entered rebel-held Moadamiyet al-Sham, encircled by government forces, and another 31 were to follow later.
- Thousands could die of starvation -
UN humanitarian coordinator Yacoub El Hillo said he hoped the relative calm would allow aid to be distributed to 154,000 besieged people over the next five days.
And the UN's food agency said "unprecedented" pledges meant it could fully fund food aid to Syrian refugees in Jordan, Lebanon, Iraq, and Egypt throughout the year.
Pressure was building to relieve civilians under siege after the UN's human rights chief said "thousands of people risk starving to death".
"The deliberate starvation of people is unequivocally forbidden as a weapon of warfare. By extension, so are sieges," Zeid Ra'ad Al Hussein said.
The flow of much-needed assistance could also create a more favourable backdrop for peace talks that collapsed in acrimony in early February.
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UN envoy Steffan de Mistura aims to relaunch negotiations on March 7 if the ceasefire lasts and more aid is delivered.
The opposition's HNC on 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 spokesman Salem al-Meslet.
An HNC letter to Ban accused the Syrian regime and its allies of committing two dozen truce violations that had killed 29 people.
The ceasefire does not apply to territory held by the Islamic State group and Al-Qaeda affiliate Al-Nusra Front.
Turkish armed forces on Monday fired 50 to 60 shells on IS positions in Aleppo province, local media reported.
Turkey's President Recep Tayyip Erdogan said the truce is "in place in one third of Syria, not all over".
"Now there's no ceasefire all over Syria. Attacks are unfortunately continuing here and there."
Russia, which has waged a five-month bombing campaign to support President Bashar al-Assad, on Sunday accused "moderate" rebels and jihadists of nine ceasefire violations.
In Aleppo, the Syrian Observatory for Human Rights reported some rebel rocket fire on government-held neighbourhoods but no casualties.
"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 in the school of the rebel-held neighbourhood of Bustan al-Qasr.
The Observatory reported nine Russian air strikes on a town in the central province of Hama but had no immediate word on casualties.
It said seven civilians were killed in Russian strikes Sunday on a town in Aleppo province where Al-Nusra has a presence.
US Defense Secretary Ashton Carter Monday stressed there had been no let-up in the US-led air campaign against IS since the truce took effect.
"There's certainly no cessation of hostilities there," he said.
The coalition said on Sunday it carried out 12 strikes against the jihadists in Syria.
Separately, activists said IS had killed eight Dutch members of the jihadist group whom it accused of trying to desert in northern Syria.
The eight were killed Friday in Raqa province, where tensions have boiled over the past month between 75 Dutch jihadists and IS intelligence operatives from Iraq, according to the citizen journalist group Raqa is Being Slaughtered Silently.