WASHINGTON—Russian personnel are engaging in potentially dangerous standoffs with U.S. troops on highways in northeastern Syria in violation of agreements between Washington and Moscow to stay out of one another’s way in the war-ravaged country, the top U.S. envoy to Syria said Wednesday.

Photos circulating on social media have shown U.S. military trucks blocking civilian vehicles that local journalists say are carrying Russian contractors on the highways. The encounters, while tense, haven’t involved the use of force.

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