Story highlights The coalition believes the attackers are part of the Turkish-backed opposition forces

The incident occurred around the same time US Secretary of Defense James Mattis visited Turkey

(CNN) US troops in northern Syria came under direct attack last week by Turkish-backed rebels, a military official with the coalition fighting ISIS told CNN Tuesday. The official said that while US troops returned fire there were no casualties on either side.

The coalition believes the attackers are part of the Turkish-backed opposition forces, a loose grouping of Arab and Turkmen fighters that have helped the Turkish military clear ISIS from the Turkish-Syria border area.

"Recent incidents have occurred in territories primarily under the control of Turkish-backed fighters. We are engaged with Ankara and other parties to address this danger," Pentagon spokesman Eric Pahon told CNN.

Many of these Turkey-supported forces originated as part of the opposition to Syrian President Bashar al-Assad and they have also clashed with the US-backed and Kurdish-led Syrian Democratic Forces in a struggle for influence in the region.

While Turkey has in the past backed these fighters with tanks, airstrikes and special forces, the coalition official made it clear that Turkish soldiers were not involved in the recent attacks on US personnel.

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