An officer in the US army and two fighters from the Kurdish-led Syrian Democratic Forces (SDF) have been killed in an ambush in Syria's eastern province of Deir Ez-Zor.
The gunmen are reported to have been from the Daesh terrorist group.
The ambush took place in the town of Al-Sur, the state-run Syrian Arab News Agency (SANA) tweeted yesterday.
The identity of the attackers was not known in the first instance. The American officer was apparently transferred to the "illegal" US military base at Al-Shaddadi in the south of the province of Al-Hasakah, located in the north-east of Syria.
Throughout the ongoing nine-year Syrian civil war, the US-led coalition has assisted and backed the SDF and other Kurdish militias militarily in their fight against Daesh.
Following the third Turkish military incursion into northern Syria – Operation Peace Spring – in October last year, in which Turkey aimed to establish a 30km safe zone for displaced Syrians, the US forces withdrew from the area.
After an agreement was struck between Turkey and Russia and the military operation ceased, however, US forces moved back into the areas from which they had withdrawn and re-established their bases.
The alliance between the US military and the SDF then continued with America securing the oil fields in Deir Ez-Zor province.
The US military presence is regarded as an illegal occupation by the Syrian regime of President Bashar Al-Assad. He aims to retake control over all of his country's territory which has been lost in the conflict.