Unidentified warplanes struck rebel-held areas in Syria's divided city of Aleppo for the first time since a fragile ceasefire took effect last week, a monitoring group reported Sunday, bringing the truce brokered by the US and Russia closer to collapse.
The aircraft fired rockets at four opposition districts in the northern city of Aleppo, leaving an unspecified number of people injured, the Syrian Observatory for Human Rights added.
Meanwhile, in the southern province of Daraa, government helicopters dropped barrels packed with explosives on a rebel-held area, killing at least eight civilians, the watchdog added.
The violence comes amid a bitter row between Moscow and Washington over the US-led airstrikes that killed dozens of Syrian regime troops on Saturday in the eastern province of Deir al-Zour.
Russia said the coalition's bombardment was on the edge of "criminal negligence and direct indulging of IS [Islamic State] terrorists," the Russian Foreign Ministry said in a TASS state news agency report.
Saturday's strikes in Deir al-Zour left 90 Syrian soldiers dead, the Observatory reported.
The coalition pilots believed they were striking Daesh targets, the US said, adding that Russia had been notified earlier in the day at its plans to operate in the area but that "no concerns were voice."
Damascus said that the attack had allowed Daesh to advance in the area, but government forces, backed by Russian air power, later recaptured the positions they had lost to militants, the Observatory said.
Syria's state news agency SANA reported that government jets on Sunday unleashed intense strikes against gatherings of Daesh fighters in Deir al-Zour and killed "dozens" of them.
Daesh militants had shot down a regime MiG fighter in the area and its pilot was killed, the Observatory reported.
SANA referred to the incident, saying a warplane crashed in Deir al-Zour while it was on a combat mission against Daesh extremists. The agency gave no details.
The rise in violence and escalating tensions between the US and Russia have cast further doubts over the shaky ceasefire that went into effect in Syria on September 12.
Russia, a key ally of Syrian President Bashar al-Assad, renewed on Sunday its call for the US to influence rebel groups it supports inside Syria, blaming them for violating the ceasefire.
"Once again, we are calling persistently on Washington to influence accordingly the illegal military formation, it patronizes, to observe fully the ceasefire regime," the Russian Foreign Ministry said.
"Otherwise, the entire complex of the Russian-US agreements reached in Geneva on September 9 will be threatened, thus contradicting interests of the entire international community," the ministry added, according to TASS.
The truce, part of an ambitious plan aimed at halting fighting in Syria and relaunching a UN-sponsored peace process, excludes Daesh and al-Qaeda extremists.
In February, a ceasefire, brokered by the US and Russia, went into effect in Syria, but did not hold for long.
By Khalil Hamlo and Ramadan Al-Fatash
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