A drone attack on a base housing US troops in eastern Syria killed six allied Kurdish fighters late Sunday, in the first significant attack in Syria or Iraq since the US launched retaliatory strikes over the weekend against Iran-backed militias that have been targeting its forces in the region.

The US-backed, Kurdish-led Syrian Democratic Forces said Monday the attack hit a training ground at al-Omar base in Syria's eastern province of Deir el-Zour and accused "Syrian regime-backed mercenaries" of carrying out the attack. No casualties were reported among US troops.