WASHINGTON — As pro-regime forces move to recapture the largest city in eastern Syria, the US-led coalition fighting the Islamic State (IS) is considering extending cooperation with Russia to avoid run-ins with its Syrian ally near militant strongholds.

Earlier this week, Syrian forces supporting President Bashar al-Assad began operations with Iran-backed troops to break a three-year siege by some 2,500 IS fighters in Deir ez-Zor, a critical thoroughfare on the Euphrates River. While the United States does not maintain diplomatic ties with Assad, a spokesman for Operation Inherent Resolve said the Pentagon could expand “de-confliction” agreements with Russia to prevent clashes with pro-regime forces.

“There is a de-confliction line that runs south of Taqba and goes all the way across … parallel to the Euphrates River, so that we can maintain our focus, both the Russians and the regime and the coalition … on defeating [IS]," Col. Ryan Dillon said in response to a question from Al-Monitor at a Pentagon press briefing on Thursday. As pro-Assad forces "move closer into the Middle Euphrates River Valley, like into Deir ez-Zor, and we de-conflict with the Russians … we'll continue to draw that line farther on down the Middle Euphrates River Valley, if necessary."

Washington and Moscow have used a de-confliction communications line that extends from a US base in Qatar to Russian command centers to warn one another of pending airstrikes in Syria, including a Russian cruise missile strike against IS from Mediterranean-based warships in June.