Co-sponsors of the ceasefire, Russia and the United States, have exchanged accusations of his attrition, with relations further strained after the US-led raid killed dozens of Syrian soldiers on Saturday.
Syria accused the United States of seeking the failure of the ceasefire.
“The goal of this American aggression is to bring the truce to a halt,” said Bashar Jaafari, Syria’s ambassador to the United Nations.
Sunday was the deadliest day of the truce so far, the Syrian Observatory for Human Rights said, with 11 civilians killed in areas where the ceasefire was supposed to take place.
Russia said the Aleppo was “particularly tense”, blaming the instability of the rebels.
“The amount of bombing by rebel groups against the positions of Syrian government troops and residential areas is increasing,” said Defense Ministry spokesman Igor Konashenkov.
French Foreign Minister Jean-Marc Ayrault accused the government of Syrian President Bashar al-Assad of undermining the ceasefire.
“We must not forget that it is first and foremost the (Syrian) regime, and it is always the regime that has jeopardized the US-Russia ceasefire,” he said.
US-led “intentional” strike?
The evening strikes in Aleppo killed one woman and injured others, said the UK-based Observatory, which was unable to identify who carried out them.
An AFP correspondent in Aleppo’s Karam al-Jabal district saw several children injured after a raid.
Ten people, including a child, were killed on Sunday when a couple of barrel bombs struck an opposition-controlled town in the southern province of Daraa, the Observatory said.
“Today was the highest death toll since the truce began,” said Observatory director Rami Abdel Rahman.
The deaths have brought the number of civilians killed across Syria on Sunday to 11 and up to 26 since the ceasefire began on Monday, he added.
Tensions between the US and Russia escalated after the US-led coalition raid that killed dozens of Syrian soldiers.
A senior Assad adviser said on Sunday that Damascus believes the strike was “intentional”.
“None of the facts on the ground show that what happened was a mistake or a coincidence,” Buthaina Shaaban told AFP.
The Observatory said at least 90 soldiers were killed in the attack on a strategic hill near Deir Ezzor. Moscow put the death toll at 62.
The Syrian army has been fighting an offensive by the Islamic State group around the key air base of Deir Ezzor since last year.
On Sunday, IS said it shot down a Syrian warplane near the city.
State media confirmed that a plane was shot down and its pilot killed, but did not say who was responsible.
‘Bad omen’ for the ceasefire
Hours after the coalition strike, the Pentagon admitted that US-led pilots may have hit Assad’s forces, but said they “believed they were targeting a Daesh (IS) fighting position.”
He said coalition forces “would not intentionally target a well-known Syrian military unit”.
Russia said it was “deeply concerned”, warning that Washington should have curbed the rebels fighting against Assad “otherwise, the realization of the Russia-US agreement … could be endangered”.
“The actions of the pilots – if, as we hope, were not undertaken on Washington’s orders – fall between criminal negligence and directly indulging the IS terrorists”, it reads.
An emergency meeting of the United Nations Security Council called by Moscow to discuss the attack saw an exchange between the United States and Russia reminiscent of the verbal jousting of the Cold War era.
US Ambassador Samantha Power said Moscow’s request for the meeting was a “stunt”, while her Russian counterpart Vitaly Churkin accused the US of violating agreements that it would not take target army positions.
Churkin called the strike a “bad omen” for the US-Russia deal to stop the war in Syria, which has killed more than 300,000 people since it broke out in 2011.
Amid the spike of tension, food aid has reached residents of the Damascus suburb of Moadamiyat al-Sham after a government deal that granted amnesty to opposition fighters in the defeated city.
Moadamiyat al-Sham Bassam Karbuj mayor said around 7,000 packages of food were distributed “under the auspices of our Russian ally”.
Residents of the suburb make up some of the 590,000 people who according to the United Nations live under siege in Syria.
Karbuj told AFP that the Syrian army would have full control of the city “in the next few days” after the remaining rebels were driven away by bus.
Meanwhile, the head of the Fateh al-Sham Front, which changed its name from Al-Nusra Front after breaking ties with Al-Qaeda, said negotiations were underway to unite the anti-regime groups into one organization. .
The merger suggested by Abu Mohamed al-Jolani would create a major break in the US-Russia agreement, which provided for cooperation between the two world powers against jihadists, including Fateh al-Sham and IS, if the truce were to last a week.