One of Russia’s top generals in Syria has been killed in an attack by the Islamic State militant group (ISIS), with Moscow blaming the U.S. for his death as it battles to defeat the jihadist cult in its last stronghold in the country.

The Russian Defense Ministry said late Sunday that Lt. Gen. Valery Asapov "was at a command post of Syrian troops, assisting the Syrian commanders in the operation for the liberation of the city of Deir Ez-Zor." Russian state news agency Tass reported that Asapov was killed “as a result of a mortar shelling by ISIS.” Russia and Syrian government forces are waging an offensive against ISIS in Deir Ez-Zor province.

Russian officials blamed Washington’s policies in Syria for Asapov’s death. Russian Deputy Foreign Minister Sergey Ryabkov said “the death of the Russian commander is the price paid with blood for the hypocrisy of American policy in Syria,” according to Kremlin news agency RIA.

The U.S.-led coalition is bombing ISIS positions in Syria in support of a Kurdish-Arab force, while the Russian air force and Iranian forces are backing the regime of Syrian President Bashar al-Assad against moderate rebels and jihadists in a bid to wrestle back large swathes of territory across the war-ravaged country.

Moscow has said it fears the U.S. is not as focused on defeating ISIS as it should be, suggesting that Washington is allowing the militant group to remain strong. Ryabkov said that U.S. claims of being in Syria principally to battle the jihadist group were not equal to its actions on the ground, in what he called a “two-faced” Syria policy.

Moscow and Washington maintain high-level contacts to prevent conflict between their forces in Syria as the Syrian Democratic Forces, or SDF, battles to retake the eastern Syrian city of Raqqa before moving on to Deir Ez-Zor along the Euphrates river, which has often served as a dividing line between the two forces to prevent them from clashing. 

Russian Defense Minister Sergey Lavrov last week hit out at President Donald Trump’s predecessor, Barack Obama, for planting a “time bomb” between the two countries that has only served to worsen relations between them. 

“International problems are suffering from the fact that Russia and the U.S. can’t coordinate their actions because of unrelated issues,” he said, suggesting that Obama’s opposition over Russia’s support for Assad, and its incursions into Ukraine, had prevented effective cooperation.

Lavrov said any action by the U.S.-backed SDF forces that would “make it difficult” for Assad’s troops to liberate Deir Ez-Zor from ISIS “would not be tolerated.”

The SDF said on Monday that Russian jets had struck their positions near a gas field they captured in Deir Ez-Zor province last week, killing one of its fighters. Moscow denied the allegation that it struck the militia’s forces at the Conoco gas plant. The U.S.-led coalition said it could not confirm who carried out the strike.

"Russian and regime forces have mounted a treacherous attack against our forces...with artillery and aircraft,” the SDF said in a statement. "We will not stand by with our arms crossed and we will use our legitimate right to self-defense.” 

Last week, Moscow told the U.S. that it would retaliate against SDF fighters firing on Syrian government troops as the two forces continue to brush against each other in eastern Syria.