Syria's opposition has accused the government of multiple ceasefire violations on day two of a temporary truce as aid convoys were delayed from bringing supplies to desperate civilians caught up in the war.
The Syrian coalition on Sunday said Bashar-al-Assad's regime had attacked 15 rebel-held areas across the country with heavy artillery, machine gun fire, and barrel bombs.
Activists also accused Syria's ally Russia of pounding at least two villages in Aleppo province with air strikes.
Al Jazeera's Omar al-Saleh, reporting from the Turkish town of Gaziantep near the Syrian border, said the opposition described the violence as a "huge violation to the terms of the ceasefire".
"Now the Russians are saying they actually targeted members of al-Nusra Front, which is linked to al-Qaeda and of course are excluded from the terms of this deal," he said.
Meanwhile, Russia said the Syrian town of Tel Abyad had been attacked from Turkish territory with large-calibre artillery. Russia's defence ministry told the Interfax news agency on Sunday it contacted a US coordination centre in Jordan "for clarification over the shelling from Turkey".
At least 12 rebels were also reportedly killed as Syrian forces fired artillery and advanced on a northern village in Latakia province.
Claims that the precarious ceasefire, brokered by Moscow and Washington, had been breached came as vital UN aid deliveries were delayed because of logistical problems.
"We are told they hope by tomorrow [Monday] to have another convoy to similar areas that received aid in previous days, but these are not in a dire situation like areas such as [the Damascus suburb of] Daraya, which has had no aid whatsoever," Saleh reported.
"This is a logistical nightmare and you need a lot of paperwork to get into those areas."
The truce agreement among most combatants in the Syrian war came into effect early on Saturday, aiming to halt fighting for two weeks with the hope it would lead to peace talks in March.
The deal marks the biggest diplomatic push yet to end Syria's five-year war, which has killed more than 260,000 people and displaced millions from their homes.
 Russia halts air strikes as Syria truce takes hold
While scepticism has abounded from the start on whether it would hold, there was hope a cessation in fighting would allow the UN to deliver assistance to an estimated 480,000 Syrians in 17 besieged areas.
"I am actually hopeful, which is a rare thing to say in the course of this conflict," Middle East analyst Hillary Mann Leverett told Al Jazeera on Saturday.
"But to me the critically important point is this is the first time that the international community has actually tried a broad-based, almost nationwide ceasefire in Syria - it's been more than four years since this has been attempted.
"The very attempt to do so - the agreement to get this done - will at least pave the way in the interim period for aid to get to civilians in desperate need."
Leverett said the parties involved had stakes in seeing that the ceasefire held. She said it would give rebel groups time to re-arm, while government forces could consolidate their authority after recent territorial gains. 
"The incentives the parties have to stay a part of this ceasefire are important, they're real, and they're not something that will be easily reversed," said Leverett.   
The Syrian Observatory for Human Rights said on Sunday, however, that at least 180 people had been killed in the first day of the ceasefire.
Russia entered the Syrian conflict in September 2015 and its air power has played a significant role in the recent major gains by government forces