Radio Free Asia, which is funded by Washington, reported on Thursday that the diplomat was trying to visit the home of a Vietnamese dissident in the city of Hue when police stopped him.
Christian Marchant, a political officer at the US embassy in the capital Hanoi, continued to try to reach Thadeus Nguyen Van Ly, with whom he had an appointment, and was reportedly then wrestled to the ground and driven away in a police car.
"It's a clear violation of the Vienna Convention" which governs the protection of diplomats, a US official, who declined to be named, said. "We're quite concerned by it."
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