Russian trucks seen on the road heading to Deir al-Zour in eastern Syria, on Sept. 21, 2017. (Omar Sanadiki/Reuters)

U.S.-backed forces in Syria accused Russia on Monday of launching airstrikes on their positions in the country's east, where world powers and their proxies are scrambling for influence amid the collapse of the Islamic State militant group.

The Syrian Democratic Forces, supported by the United States, said Russian warplanes targeted SDF fighters in three locations in Deir al-Zour province Monday, including at a major gas field they had seized Saturday from militants. It was the second time this month that the SDF, a coalition of Arab and ethnic Kurdish fighters, has claimed it was targeted by Russian strikes.

Also Monday, Russia blamed what it called "the duplicity of U.S. policy" in Syria for the death of a Russian general killed by Islamic State mortar fire in Deir al-Zour. Lt. Gen. Valery Asapov is the highest-ranking Russian officer killed in Moscow's intervention in Syria, which began two years ago.

His death was "the price paid in blood" for U.S. hypocrisy, Deputy Foreign Minister Sergei Ryabkov said. Moscow has accused the United States of tacitly allowing the ­Islamic State to fight Syrian government forces while claiming to be Russia's ally against terrorism.

[Russia threatens retaliation against U.S. troops and allies in Syria]

Tensions between the United States and Russia in eastern Syria have intensified in recent weeks, as both the Russian-backed Syrian government and U.S.-supported proxies fight parallel battles against the Islamic State.

In Deir al-Zour, a desert region rich in oil and gas, the extremist group is struggling to hold on to one of its last redoubts in Syria. As the Islamic State's defenses crumble, the province has emerged as a key prize in the battle to shape the country's postwar future.

The SDF, bolstered by U.S. airstrikes and Special Operations forces, has advanced on Deir al-Zour from the north. On Saturday, SDF fighters seized the Conoco gas field, named after the Houston-based oil conglomerate that built and operated a plant there in the early 2000s. It was one of the Islamic State's most important sources of funding, analysts said.

For the government, such fields represent revenue and power, allowing it to reassert control in the face of years of armed rebellion. To the Kurds, whose militias dominate the SDF, the resources are a bargaining chip in their negotiations for autonomy.

Iran, a strong backer of the Syrian government, needs Deir al-Zour to secure a land corridor from Tehran to its Lebanese proxy, Hezbollah. Iranian-backed militias have assisted the government in eastern Syria. And the United States, while insisting it is uninterested in the geopolitical fight, is likely to welcome any expansion of its allies' territory as it looks to thwart Iran's regional ambitions, analysts said.

The battles are taking place amid U.S.-Russian rivalry in Syria — and across the Middle East. U.S. power in the region has been challenged by the chaos of the Arab Spring, and Russia's support for Syria's government has ensured Moscow a foothold in the region.

[How far could the dangerous endgame in eastern Syria go?]

In recent weeks, Russia has begun to take aim at the SDF and its U.S. backers. According to the state-run Rossiya-24 news channel, the Russian military believes that U.S. Special Operations forces are coordinating their moves in Syria with the Islamic State.

Russia recently staged a press tour to display its military strength in Syria, from special forces troops on the ground to a powerful naval force in the Mediterranean Sea.

At a news conference Monday, Maj. Gen. Igor Konashenkov, a Defense Ministry spokesman, said that Russian "drones and intelligence" in Deir al-Zour "have not observed any military clashes" between the SDF and the Islamic State.

The U.S.-led coalition said Monday that it was aware of reports of Russian strikes on SDF targets but called the accounts "inconsistent."

A coalition spokesman, U.S. Army Col. Tom Veale, said in a statement that the coalition "de-conflicts the complex and congested battlespace over eastern Syria with Russian officials on a regular basis."

The "communication between the coalition and the Russians ensures the safety of our collective aircrews and assets, and supports our operations on the ground."

Filipov reported from Moscow.

Read more:

Russia threatens retaliatory strikes against U.S. troops and their allies in Syria

Today's coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news