A flurry of attacks on rebel-controlled areas of Aleppo risks rebuilding fronts in the city and could be the most serious threat to a ceasefire.
The cessation of fighting around Aleppo and the delivery of much-needed humanitarian aid were key components of the delicate deal, which took effect last Monday.
Co-sponsors of the ceasefire, Russia and the United States, have exchanged accusations of disrupting it, and relations have worsened after dozens of Syrian soldiers were killed in a U.S.-led raid on Saturday.
Syria has accused the United States of seeking a ceasefire.
“The goal of this US aggression is to disrupt the ceasefire,” said Syrian Ambassador to the UN Bashar Jafari.
Sunday was the deadliest day of the ceasefire, the Syrian Observatory for Human Rights said: 11 civilians were killed in areas where a ceasefire was expected.
Russia has said Aleppo is “particularly tense”, accusing it of insurgent instability.
“The number of attacks by insurgent groups on the positions of Syrian government troops and residential areas is increasing,” said Defense Ministry spokesman Igor Kanashenkov.
French Foreign Minister Jean-Marc Héroe has accused Syrian President Bashar al-Assad’s government of undermining the ceasefire.
“We must not forget that this is primarily a (Syrian) regime, and it is always a regime that has threatened the US-Russian ceasefire,” he said.
A “deliberate” US-led strike?
As a result of the evening strikes on Aleppo, one woman was killed and others were injured, according to the British Observatory, which could not determine who inflicted them.
An AFP correspondent in the Karam el-Jabal area of Aleppo saw several injured children after the raid.
Ten people, including a child, were killed Sunday when a pair of barrel bombs hit an opposition-controlled city in the southern province of Daraa, the observatory said.
“Today was the highest death toll since the ceasefire began,” said Rami Observatory Director Abdel Rahman.
He added that the number of Syrian civilians killed on Sunday had risen to 11 and 26 since the ceasefire began on Monday.
Tensions between the United States and Russia have escalated following a US-led coalition raid that killed dozens of Syrian soldiers.
Assad’s senior adviser said Sunday that Damascus believes the strike was “deliberate”.
“None of the facts on the spot show that what happened was a mistake or a coincidence,” Butain Shaaban told AFP.
The observatory said at least 90 soldiers had been killed in a strike on a strategic hill near Deir ez-Zor. 62 people died in Moscow.
The Syrian army has been battling an Islamic State offensive around the key Deir ez-Zor air base since last year.
On Sunday, the ISIS said it shot down a Syrian military plane near the city.
State media confirmed that the plane was shot down and its pilot killed, but did not say who was to blame.
A “bad sign” for a ceasefire
Hours after the coalition strike, the Pentagon acknowledged that U.S.-led pilots may have struck at Assad’s forces, but said they “believed they were attacking ISIS (IS) combat positions.”
It says coalition forces “will not deliberately strike at a prominent Syrian military unit.”
Russia has said it is “deeply concerned”, warning that Washington will have to deter insurgents fighting Assad, “otherwise the implementation of Russian-American agreements … could be jeopardized”.
“The actions of the pilots – if they are hopefully not taken on orders from Washington – are between criminal negligence and direct indulgence of IS terrorists,” the statement said.
At an emergency meeting of the UN Security Council convened by Moscow to discuss the attack, an exchange took place between the United States and Russia, reminiscent of Cold War-era verbal knightly battles.
US Ambassador Samantha Power said Moscow’s request for a meeting was a “trick”, and her Russian counterpart Vitaly Churkin accused the United States of violating agreements that they would not target the army’s position.
Churkin called the strike a “bad omen” for the US-Russian agreement to end the war in Syria, which has killed more than 300,000 people since the 2011 bombing.
Amid rising tensions, food aid has reached residents of Damascus suburb Maadamiyat al-Sham following a government agreement to amnesty opposition fighters in the besieged city.
Maadiami Mayor al-Sham Basam Karbuj said about 7,000 food parcels had been distributed “under the auspices of our Russian ally.”
Suburban residents make up some of the 590,000 people who, according to the UN, live under siege in Syria.
Carbuj told AFP that the Syrian army would have full control of the city “in the coming days” after the remaining rebels were expelled by bus.
Meanwhile, the leader of the Front, Fateh al-Sham, who changed his name to the Al-Nusra Front after severing ties with al-Qaeda, said talks were under way to unite anti-regime groups into a single organization.
The union, proposed by Abu Mohammed al-Jalani, will play a key role in an agreement between the United States and Russia that provided for cooperation between the two world powers against jihadists, including Fateh al-Sham and the Islamic State, if the ceasefire lasts a week.