The Syrian Democratic Forces, a Kurdish-dominated militia supported by U.S.-led coalition airstrikes, said Saturday that they would clear the Islamist militants from territory east of the Euphrates River.
"We at the military council decided to start this decisive operation," said Abu Khawlah, a spokesman for the militia. Dubbed "Operation Jazeera Storm," the offensive will take place in the Khabur River valley, the coalition said in a separate statement, adding that it planned to hand the area over to a civilian council.
"The morale of our forces is strong and we are ready for victory," said Khawlah.
But in one Syria's most complex battlefields, that achievement is far from assured. Government forces backed by Russian warplanes and Lebanese militia reached Deir al-Zour city this week, lifting an almost three-year Islamic State siege and boosting President Bashar al-Assad's argument that his forces should retake the country's final Islamic State-held pockets.
According to the Syrian Observatory for Human Rights monitoring group, pro-government troops continued their advance Saturday, shelling Islamic State positions while Russian planes launched bombing raids.
In Damascus, Syria's foreign ministry said the advance "foreshadowed the end of terrorism."
With SDF forces in control of more than 65 percent of the Islamic State's de facto Syrian capital, Raqqa, and most of the militant group's Iraqi strongholds recaptured, Deir al-Zour province has become the Islamic State's most important refuge.
Leadership figures are understood to have been sighted in its southern cities of Mayadin and Bukamal, among them the group's most senior leader, Abu Bakr al-Baghdadi.
Although U.S. officials have insisted that they do not anticipate clashes with the Syrian-backed force, the growing complexity of the battleground could make unintended flash points more likely.
A foothold in Deir al-Zour province would provide Washington with an opportunity to block Iranian expansionism in a strategically important area along the Iraqi border. Victory for the Iran-backed force, meanwhile, would strengthen what has effectively become an unbroken line of control running east from its Lebanese proxies Hezbollah through Syria, Iraq and Iran.
It would also provide the Assad government with a vital economic lifeline, as the province is rich in oil wealth. Six years of war have caused some $226 billion in losses, according to the World Bank, leaving the state heavily dependent on credit lines from Russia and Iran.
In the posturing on both sides, analysts saw early attempts to set a narrative of success, even while neither appeared to be mustering adequate troop numbers to finish the job.
"Assad and his allies rushed to Deir al-Zour fully aware that he does not have a force capable of capturing all of the city or to wage an effective campaign to seize all of the province," said Nicholas Heras, a fellow at the Washington-based Center for a New American Security.
"That harsh reality doesn't matter; what matters to Assad and his allies is their ability to undermine the U.S. claim that Assad is not able, or willing, to fight ISIS."
Heba Habib in Stockholm contributed to this report.