The U.S.-backed coalition to defeat the Islamic State commenced what it says is an operation to drive the terror group out of its last remaining pocket in the Euphrates River valley of eastern Syria.

Kurdish-led Syrian Democratic Forces, Washington’s primary local ally in Syria, are moving to recapture Hajin, a town on the Euphrates about 20 miles from the Iraq-Syria border.

Hajin and the surrounding countryside are home to many of the estimated 14,000 ISIS fighters that remain in Syria.

“The operation is the last major SDF ground offensive in the MERV (Middle Euphrates Valley) to liberate the citizens from the brutal rule [of ISIS]” they have suffered “over the past four years,” Army Col. Sean Ryan, spokesman for Operation Inherent Resolve, told Kurdistan 24 News on Tuesday.

U.S. air and artillery strikes will support the SDF ground offensive, which is dubbed Operation Roundup, Ryan added in a statement on Twitter.

The last phase of the operation comes after SDF fighters cleared ISIS militants out of Deir ez-Zior city and surrounding areas in August. A victory in Hajin would give coalition forces complete control of all the urban centers straddling the Euphrates River from Raqqa south to the Iraqi border.

ISIS first overran much of northern and western Iraq and eastern Syria in 2014, capturing sparsely populated territory and major urban areas including Mosul — Iraq’s second largest city — and Raqqa, the self-declared capital of its “caliphate.” After two years of setbacks, a U.S.-backed coalition began to swiftly take back much of the ISIS-held territory, culminating with the liberation of both cities by the end of 2017.

As coalition forces have squeezed the group’s positions in eastern Syria, many ISIS militants have fled across the border into Iraq. Between 15,000 and 17,000 ISIS fighters are based in Iraq, according to UN estimates — mostly in the country’s vast western desert or in mountainous areas of Iraqi Kurdistan. (RELATED: UN Report: There Are Still As Many As 30,000 ISIS Fighters Scattered Across Iraq And Syria)

Tuesday’s offensive on Hajin comes as Washington is broadening the scope of U.S. military intervention in Syria beyond the mission to destroy ISIS. The “resounding defeat” of the group remains a priority, but American troops will remain in Syria until all Iranian proxy forces have left the country and Damascus is controlled by a stable, non-threatening government, Syria envoy James Jeffrey said Sept. 6. (RELATED: Trump’s Syria Envoy Confirms US Commitment To Another Open-Ended War In Middle East)

An indefinite military commitment in Syria would reverse the previous position taken by President Donald Trump, who said in April that U.S. troops would withdraw from the country “very soon” after the defeat of ISIS.

Follow Will on Twitter

Send tips to will@dailycallernewsfoundation.org.

Content created by The Daily Caller News Foundation is available without charge to any eligible news publisher that can provide a large audience. For licensing opportunities of our original content, please contact licensing@dailycallernewsfoundation.org.