Russia's Foreign Ministry has said that "an object similar to an unarmed mortar shell" was confiscated from the baggage of a U.S. Embassy employee at Moscow's Sheremetyevo Airport on March 9, the Russian state news agency TASS reported.
The report said the object had a detonator but no explosives.
According to the TASS report, the Russian Foreign Ministry considered the incident "a provocation."
The incident comes at a time of tense relations between the two countries, aggravated last month by a U.S. decision to withdraw from a key Cold War-era nuclear-arms treaty amid accusations that Moscow had violated it.
In response, Moscow also suspended its participation in the 1987 Intermediate-Range Nuclear Forces (INF) Treaty and accused the United States of violating it.
"The U.S. Embassy was immediately informed about the incident," an unnamed Foreign Ministry source was quoted as saying.
The unidentified embassy employee was allowed to board a flight to New York after the object was confiscated.
The U.S. Embassy has not commented on the alleged incident.