Story highlights Levi Jonathan Shirley died fighting ISIS in northern Syria, YPG says

United States has not publicly confirmed his death

(CNN) An American civilian fighting alongside Kurdish troops in Syria has been killed fighting ISIS, according to a statement from the official website of the People's Protection Units, or YPG.

The YPG said Levi Jonathan Shirley, a volunteer also known as Agir Servan, died on July 14 in Manbij, an ISIS-held city in northern Syria.

Shirley first traveled to Syria in February 2015, joining the YPG in Rojava. After a short trip back to the United States, he returned to Syria in January 2016, fighting in Al-Jazira and Kobani before the offensive to recapture Manbij.

The YPG, a Kurdish group of some 30,000 fighters, is the senior partner in the Syrian Democratic Forces, which also includes some smaller Arab and Christian groups.

Image of deceased fighter Levi Jonathan Shirley released by the YPG.

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