Russian President Vladimir Putin has questioned the US commitment to a shaky ceasefire in Syria suggesting that Washington wasn't prepared to break with "terrorist elements" battling Syrian President Bashar al-Assad's forces.
The truce came into effect on Monday but both sides have alleged dozens of violations, and aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo - a key opposition demand.
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Russia has in turn accused Washington of failing to rein in the rebels, and on Saturday Putin asked why the United States has insisted on not releasing a written copy of the agreement. Officials have provided details of the deal in press conferences, but have not released an official document, fuelling suspicions on both sides.
"This comes from the problems the US is facing on the Syrian track - they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements," Putin said during a trip to Kyrgyzstan.
"In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route."
He appeared to be referring to Jabhat Fateh al-Sham, formerly known as al-Nusra Front, which is deeply embedded in rebel-held areas and fights alongside more moderate groups.
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Russian Foreign Minister Sergey Lavrov echoed Putin's remarks during a phone call with US Secretary of State John Kerry.
Lavrov noted the "refusal by an array of illegal armed groups to join the ceasefire", and Washington's obligation to "separate units of the moderate opposition from terrorist groupings", according to a Foreign Ministry statement.
Al Jazeera's Mohammed Jamjoom, reporting from Moscow, said Russia's Defence Ministry noted on Saturday that if there was a collapse of the ceasefire, it would be the fault of the United States.
"According to Russian officials there have been almost 200 ceasefire violations and they blamed all of those violations on opposition forces in Syria," Jamjoom said.
Under the ceasefire agreement, the US and Russia would work together to target Jabhat Fateh al-Sham, as well as Islamic State of Iraq and the Levant (ISIL, also known as ISIS), while Assad's forces refrain from striking opposition-held areas.
But Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint coordination centre.
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The leader of Jabhat Fateh al-Sham, Abu Mohammad al-Golani, condemned the US and Russian-brokered ceasefire, saying it places Washington on the side of President Assad and against the Syrian people.
In an interview with Al Jazeera that was broadcast on Saturday, al-Golani said that the ceasefire deal reached last week would allow Assad's forces to advance, while eradicating any substantial armed challenge on the ground.
Golani warned that "if the US takes this step, it will be in conflict not only with one faction, but with the whole Syrian people".
Syria's conflict has killed more than 300,000 people and displaced half the country's population since March 2011.
Source: Al Jazeera News And Agencies