A fighter from Deir al-Zour military council, which fights under the Syrian Democratic Forces (SDF), in the village of Abu Fas, in Hasaka province. (Rodi Said/Reuters)

The U.S.-led coalition fighting the Islamic State in Syria said Saturday that its partner forces have been attacked by Russian warplanes, escalating tensions on one of the country's most complex and contested battlefields.

The coalition said in a statement that an early morning airstrike targeted positions used by the Syrian Democratic Forces, or SDF, a Kurdish-dominated militia backed by Washington, as well as the international advisers supporting them in a weeks-old offensive to dislodge Islamic State militants from the eastern province of Deir al-Zour.

As the Islamic State loses territory across Syria and Iraq, oil-rich Deir al-Zour has become a hub for the group’s senior leaders.

Stretching along the eastern border with Iraq, it has also emerged as a geopolitical battleground for forces trying to support or thwart Iranian attempts to secure supply routes stretching from Beirut to Tehran.

The U.S.-led coalition intervened in Syria and Iraq in 2014 to halt the Islamic State’s conquest across swaths of both countries. Iran’s and Russia’s involvement dates back to the early months of the civil war that followed Syria’s 2011 anti-government uprising, bankrolling and later militarily supporting President Bashar al-Assad’s forces as part of a broader attempt to maintain key allies across the region.

In Deir al-Zour, the two military groups find themselves fighting a common enemy. SDF fighters have advanced against Islamic State positions on the eastern bank of the Euphrates River in recent weeks, while forces allied with Assad’s government have cut through the extremist group’s territory from the western side, entering Deir al-Zour city and bringing the rival coalitions to within several miles of each other.

Saturday’s attack marks the first time that the U.S.- and Russian-backed forces have clashed directly in the province, signaling what appeared to be a growing willingness on the part of Russia to protect its pro-Assad allies as they consolidate control over the most strategic areas.

Although the Pentagon, SDF, Russia and Syria have agreed on a line of “physical separation” between their parallel offensives, tensions have been rising in recent days.

“Coalition officials are available and the deconfliction line with Russia is open 24 hours per day,” said coalition commander Lt. Gen. Paul E. Funk II in a statement Saturday.

“We put our full efforts into preventing unnecessary escalation among forces that share ISIS as our common enemy,” Funk said, adding that the coalition and its partners retained a right to self-defense.

The skies over Syria have become increasingly congested as the six-year conflict has dragged on, with warplanes from the coalition, the Syrian government and Russia all carrying out airstrikes.

Although direct clashes are rare, the U.S.-led coalition launched strikes in early June against pro-Assad militiamen in southern Syria, after they advanced on an outpost housing American Special Forces, as well as a contingent of rebel fighters they supported.

On Friday, Ahmed Abu Khawla, a leading SDF commander, warned the Syrian army and allied militias against firing across the Euphrates as his fighters closed in — something he said had happened in recent days.

“We have notified the regime and Russia that we are coming to the Euphrates riverbank, and they can see our forces advancing,” he told the Reuters news agency.

In an interview with Al Manar TV, a channel affiliated with the Lebanese Hezbollah movement, a prominent Assad adviser insisted Friday that pro-government forces reserved the right to attack.

“Whether it’s the Syrian Democratic Forces, or Daesh or any illegitimate foreign force in the country . . . we will fight and work against them so our land is freed completely from any aggressor,” she said, using an Arabic acronym for the Islamic State.

Zakaria Zakaria in Istanbul contributed to this report.

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