Ceasefire teeters on brink

Syria's ceasefire was hanging by a thread yesterday, 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.

Moscow stepped up its war of words with Washington yesterday, saying the air strikes threatened the implementation of a US-Russian ceasefire plan for Syria and bordered on connivance with Islamic State.

Russia has repeatedly called on the United States to push units of moderate Syrian opposition to separate from Islamic State and other "terrorist groups".

The Foreign Ministry said Saturday's incident was a result of Washington's "stubborn refusal" to cooperate with Moscow in fighting Islamic State, the Nusra Front - now renamed Jabhat Fatah al Sham - and "other terrorist groups".

The Syrian Observatory for Human Rights said at least 90 soldiers were killed in the strike. 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 yesterday and fought back control of area around the key air base.

Yesterday IS said it shot down a Syrian warplane near Deir Ezzor. Syrian state news agency SANA confirmed a plane had been shot down there and its pilot killed, but did not say who was responsible.

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.

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."

A statement by Russia's foreign ministry yesterday said it was "deeply concerned" about the incident.

"The actions of the pilots -- if they, as we hope, were not taken on orders from Washington -- fall between criminal negligence and direct pandering to IS terrorists," it said.

It again called on Washington to force Syrian rebels to sign on to the truce, warning that "otherwise, the realisation of Russia-US agreements... could be put in danger."

An emergency UN Security Council meeting called by Moscow to discuss the attack ended early on Saturday after an exchange between the US and Russia reminiscent of Cold War-era verbal jousting.

Washington further unnerved Moscow when its envoy to the United Nations abruptly left her seat as the Russian representative took the floor to condemn the air strikes.

US ambassador Samantha Power said Washington regretted the loss of life, but that Moscow's request for the meeting was a "stunt".

And 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 the US-Russia deal on halting the war in Syria, which has killed more than 300,000 people since it erupted in March 2011.

"We are reaching a really terrifying conclusion for the whole world: That the White House is defending Islamic State. Now there can be no doubts about that," Russian Foreign Ministry spokeswoman Maria Zakharova said in comments aired by state TV. Samantha Power retorted that Zakharova should be embarrassed by that claim.

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.