He deferred questions to the Justice Department and added, "The coalition's mission is to defeat ISIS in Syria and Iraq, and we will pursue ISIS fighters regardless of nationality".
The group is supported by United States weapons, air strikes and intelligence as they battle IS in Syria, where the fight is now focused on Raqa and the Euphrates River valley.
US-backed fighters of the Kurdish-led Syrian Democratic Forces are making their final big push to drive ISIS militants from Raqqa city's western districts.
REUTERS/STRINGERA loyal ISIS fighter runs around the streets of Syria.
Asked earlier Thursday about the reports of an American captured on the battlefield, a spokesman for the USA -led coalition fighting ISIS said it is committed to combatting the group no matter what nationalities its members hold.
He said Islamic State had "shown fierce resistance" when SDF fighters entered the outskirts of Deir al-Zor on the eastern bank of the Euphrates.
The announcement about the offensive in Syria came hours after an Iraqi military commander said troops have launched a multi-pronged operation to capture a town near the Syrian border from IS. Although they do monitor any attempts to support the terrorist groups of the Islamic State or al-Qaeda.
The regime and SDF advances are bringing the two forces into increasing proximity along the Euphrates, with Dillon saying Thursday that "increased" de-confliction was needed between Russian Federation and the coalition as a result of the two sides being so close to one another.
Coalition military forces, he said, conducted three strikes consisting of 13 engagements against ISIS targets on September 14 in Huwayjah and Rawah.
Dillon tweeted a photograph of coalition medics treating wounded SDF soldiers.
"Coalition officials are available and the deconfliction line with Russian Federation is open 24 hours per day", Army Lt. Gen. Paul E. Funk II, commander of the coalition forces, said Saturday in the press release.