The US backed Syrian Democratic Forces launched a ground offensive Monday to clear ISIS from its last remaining territory in Syria, the US-led coalition announced Tuesday.
"We'll see how tough the fight is," Secretary of Defense James Mattis said of the offensive against the last ISIS redoubt east of the Euphrates River.
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"I wouldn't be surprised if some of ISIS' leadership is up there," Mattis added when asked if senior ISIS leaders involved in foreign plots could be based there.
The offensive, which the US-led coalition fighting ISIS has called phase three of Operation Roundup, is in an area centered around the town of Hajin which sits along the Euphrates River some 15 miles from the border with Iraq.
"Operations will clear ISIS remnants from Hajin and the remaining Deir-Ez-Zoir countryside east of the Euphrates River," the coalition said in its statement.
The offensive by the Syrian Democratic Forces, a mix of Kurdish and Arab fighters, will be backed by coalition cross-border air and artillery strikes.
The deputy commander of the coalition, UK Maj. Gen. Felix Gedney, told reporters last month that ISIS commanded "over a thousand" fighters in the area around Hajin.
On Sunday, coalition military forces conducted three strikes near Hajin, targeting ISIS tactical units and destroying an ISIS staging area and command and control center according to the coalition.
The Pentagon told CNN that coalition forces carried out 10 additional strikes Tuesday in support of the offensive.
The US military assesses that ISIS has lost over 98% of the territory it controlled at its peak, an area that once stretched across Syria and Iraq and included major cities like Mosul and Raqqa.
The US-led air campaign has also destroyed most of the terror group's revenue streams and killed numerous leaders involved in planning terror attacks around the world.
However, a recent Inspector General report said that despite its territorial losses, ISIS still commands up to 30,000 fighters, and the Pentagon has said that the terror group is "well positioned" to rebuild.
US military planners have said that many of the approximately 2,000 US troops in Syria remain in order to ensure the "enduring" defeat of ISIS by helping to train thousands of local security forces.