Beirut: Syria's ceasefire was hanging by a thread on Sunday, after tensions escalated between Moscow and Washington over a US-led coalition air strike that killed dozens of Syrian soldiers battling jihadists.
The truce saw violence drop across Syria for several days after it came into force Monday, but fighting has since erupted on several fronts and besieged civilians are still waiting desperately for promised aid deliveries.
The ceasefire`s co-sponsors, Russia and the United States, have traded accusations over the fraying deal, with relations strained even further after the US-led raid killed scores of Syrian soldiers on Saturday.
The Syrian Observatory for Human Rights said at least 90 soldiers were killed in the strike. Damascus ally Moscow put the death toll at 62.
The Syrian army said the raid had allowed Islamic State group fighters to gain ground around the key eastern airbase of Deir Ezzor, but a military source said government forces were back on the offensive on Sunday.
"The army has retaken most of its positions on Jabal Therdeh with Russian and Syrian air support," the source said.
He was referring to hilltop positions that overlook the airbase. Retaking them is vital for the army to prevent IS using them to fire on aircraft taking off from or landing at the base.
The base and adjacent government-held neighbourhoods of Deir Ezzor city have been under siege since 2012 and have been dependent on resupply by air.Observatory head Rami Abdel Rahman said Russian warplanes pounded IS positions around Deir Ezzor late Saturday as fierce clashes broke out between IS and forces loyal to President Bashar al-Assad.
At least 38 jihadist fighters were killed and clashes were ongoing, Abdel Rahman told AFP.
On Sunday, the IS-linked Amaq news agency said IS shot down a Syrian warplane. Abdel Rahman confirmed the attack and said the pilot was killed.
Hours after the coalition strike on Saturday, the Pentagon admitted US-led pilots may have hit Assad`s forces but said that they "believed they were striking a Daesh (IS) fighting position."
The statement said coalition forces "would not intentionally strike a known Syrian military unit" and that an investigation was underway.
Australia, which said it was one of several coalition countries whose aircraft took part, offered its "condolences to the families of any Syrian personnel killed or wounded."
Russia called an emergency UN Security Council meeting late Saturday to demand an explanation from the US.
US ambassador Samantha Power said that Washington regretted the loss of life, but that Moscow`s request for the meeting was "a "stunt, replete with moralism and grandstanding."
In an exchange reminiscent of Cold War-era verbal jousting, Russian ambassador Vitaly Churkin accused the US of violating agreements that it would not target Syrian army positions.
He called the strike a "bad omen" for a US-Russian deal on halting the war in Syria, which has left more than 300,000 people dead since it erupted in March 2011.
"These strikes endanger everything that has been done so far by the international community" to end the conflict, foreign ministry spokeswoman Maria Zakharova said. The truce agreement -- negotiated by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov -- came into force on Monday evening.
The Britain-based Observatory, which has a broad network of sources across Syria, reported a drop in violence across all major fronts in the country where IS was not present.
But fighting began to build up again late last week, including in the central provinces of Homs and Hama, and the rebel-held eastern suburbs of Damascus.
The front line remained quiet in divided second city Aleppo, but civilians have yet to see any aid deliveries -- a key component of the truce deal.
The head of Fateh al-Sham Front, which changed its name from Al-Nusra Front after renouncing its ties to Al-Qaeda, said late Saturday that "neither we nor rebel groups will allow the siege of Aleppo to continue."
In an interview broadcast by Al-Jazeera television, Abu Mohamed al-Jolani said negotiations were under way for anti-regime groups to band together in a single organisation.
Such a merger would throw a major wrench in the US-Russia deal, which foresaw cooperation between the two world powers against jihadist groups, including Fateh al-Sham as well as IS, if the truce holds for a week.
