Rapid advances by Russian- and Iranian-backed government forces in eastern Syria are thwarting the U.S. military’s hopes of pressing deeper into Islamic State territory after winning the battle for Raqqa.

An expansion of territory held by forces loyal to President Bashar al-Assad is also likely to provide Assad with additional leverage in political negotiations over Syria’s future, talks the United Nations hopes to reconvene next month.

In a statement this week, U.N. Secretary General António Guterres said the “latest developments” in Syria pointed “to the urgent need to reinvigorate the political process.”

The recent government gains have cut off the approach of the U.S.-backed Syrian Democratic Forces to remaining militant strongholds in the southeastern part of the country, including the crucial town of Bukamal near the Syria-Iraq border.