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.

Top U.S. envoy James Jeffrey confirmed such standoffs were occurring in northeast Syria with increasing frequency. Some have taken place deep in areas patrolled by U.S. and Kurdish forces, where Russian contractors aren’t expected to go.

“These are not daily occurrences, but they have been increasing in number, and thus it is troubling,” Mr. Jeffrey told reporters here on Wednesday.

The incidents are raising concerns in Washington about the risk of a clash between the two major powers as Russia-backed Syrian troops escalate an offensive on the last remaining rebel enclaves in Idlib province, home to about three million people, including many displaced from other parts of Syria.