Russia has ordered an alleged agent of the United States' Central Intelligence Agency (CIA) to leave the country.
The Russian Foreign Ministry said on May 14 that the man, identified as Ryan Christopher Fogle, must return to the United States "as soon as possible."
It added that "such provocative actions in the spirit of the Cold War will by no means promote the strengthening of mutual trust" between Moscow and Washington.
Russia's Federal Security Service earlier said Fogle was detained overnight for allegedly trying to recruit a Russian counterterrorism officer who specializes in the volatile Caucasus region in southern Russia.
The region includes Chechnya, where the two Boston Marathon bombing suspects had their ethnic roots.
Russian state TV showed pictures of the man it identified as Fogle. It also displayed objects said to belong to him, including two wigs, a compass, a map of Moscow, a pocket knife, three pairs of sunglasses, and envelopes containing 500 euro notes.
The report said Fogle worked "undercover" as third secretary of the U.S. Embassy's political section.
The State Department on May 14 only confirmed that Fogle worked as an embassy employee but wouldn't give any details about his employment record or responsibilities in Russia.
"We can confirm that an officer of the U.S. Embassy in Moscow was briefly detained and was released. We have seen the Russian Foreign Ministry announcement and we have no further comment at this time," a U.S. State Department spokesperson, Patrick Ventrell, told reporters in Washington.
The Russian Foreign Ministry summoned U.S. Ambassador Michael McFaul to appear on May 15 in connection with the case.
McFaul said he would not comment on the spying allegation. He also declined to comment on the case during a Q&A session on Twitter on May 14.
Despite the end of the Cold War, Russia and the United States still maintain active espionage operations against each other.
Fogle was the first American diplomat to be publicly accused of spying in Russia in about a decade.
Last year, several Russians were convicted in separate cases of spying for the United States and sentenced to lengthy prison sentences.
With reporting by Interfax, AP, Reuters, mid.ru, and AFP