Israel, alarmed about the clout of arch enemies Iran and Hezbollah, has pressed Russia, Assad's other key ally, to make sure Tehran does not entrench its military sway in Syria.
The Syrian government repeatedly denounced the criminal attacks of the Western Coalition forces, led by the United States, against the civilian population and positions of the army, which killed hundreds of residents and soldiers.
The Syrian Observatory for Human Rights, a Britain-based monitor of the conflict, said it was one of the deadliest air attacks on government loyalists in recent months.
The death toll is expected to rise, the British-based watch group said.
The bodies of three Iraqi fighters killed in the raid were returned on Monday to their hometowns for burial, said AFP's correspondent in the southern Iraqi city of Nasiriyah.
Syrian state media, citing a military source, reported on Monday that USA -led coalition aircraft had bombed "one of our military positions" in eastern Syria, leading to deaths and injuries, but the US military denied carrying out strikes in the area.
"No member of the U.S. -led coalition carried out strikes near Albu Kamal", U.S. Central Command spokesman Major Josh Jacques told Reuters.
The report, quoting an unnamed military official, gave no breakdown of the casualties other than saying there "were several martyrs and others were wounded".
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"At 22:00 last night a US plane hit a fixed headquarters of the Popular Mobilisation Forces' 45th and 46th brigades defending the border strip with Syria, using two guided missiles which led to the martyrdom of 22 fighters", it said in a statement. At least 52 fighters were killed in the attacks, including a number of Syrian troops.
A US official, speaking on the condition of anonymity, said the USA military was concerned that Popular Mobilisation Forces could retaliate against American forces in Iraq.
"We haven't carried out a strike in Abu Kamal", Rankine-Galloway said.
Both Syrian authorities and Iraqi forces pointed the finger at the US-led coalition, which denied it was involved in Sunday night's attack.
Regime forces control land west of the Euphrates River that runs through the province, while the SDF are battling to expel IS from a string of villages on the river's eastern banks near the Iraqi border.
The Syrian army, alongside allied Iran-backed militias including Lebanon's Hezbollah and Iraqi groups, drove Islamic State from Albu Kamal and its environs past year, but the jihadists have since staged attacks in the area. The U.S. -led coalition has struck pro-government forces in the past when they have tried to cross the river.
According to the officer, Imam Ali Brigades and Righteous Brigades members were killed in the military action, which took place south of Al-Bukamal, about 150 kilometers east of Deir Ezzor.