Russia angrily hit back yesterday at US claims that it had smashed a Moscow-organised spy ring, warning the spat could damage efforts to improve relations.
US authorities said 10 "deep-cover" suspects, accused of infiltrating policymaking for the Kremlin, had been detained for seeking details of US nuclear weapons and foreign policy.
Five accused appeared in court in New York on Monday and some of the suspects are apparently Russian nationals. According to US documents, the spy ring had been under FBI surveillance for a decade.
The Russian Foreign Ministry condemned the spy allegations made by the US Justice Department. Foreign Minister Sergei Lavrov demanded an explanation of the charges.
"In our opinion, such actions are groundless and pursue unseemly aims," a Foreign Ministry spokesman said.
"In any case, it is highly deplorable that all of this is happening against the background of the reset in Russia-US ties announced by the US administration itself."
The arrests on Sunday in four northeastern US states came only three days after President Barack Obama described his Russian counterpart Dmitry Medvedev as a "solid and reliable partner" at a White House summit.
"They did not explain what the matter is about. I hope they will," Russia's Mr Lavrov was quoted by Russian news agencies as saying during a visit to Jerusalem. "The moment when it was done has been chosen with a special finesse," he said with apparent sarcasm.
Russian foreign ministry spokesman Igor Lyakin-Frolov told AFP there were a "lot of contradictions" in the information about the case.
The White House has yet to comment on the case which harks back to Cold War hostilities with the use of false identities and tales of buried money and hidden video cameras. US authorities said an 11th suspect remains at large.
"You were sent to USA for long-term service trip," read a message decrypted by the FBI and said to be from the Moscow headquarters of the SVR intelligence service, a successor to the communist-era KGB.
Alleged 'secret agents' used Malta password
Russian secret agents arrested by the FBI yesterday, are reported to have used a "Malta" password to identify each other.
The Washington Post said SVR headquarters, Moscow Centre's instructions were explicit: For a meeting in Rome, its American spy would approach a stranger and ask, "Excuse me, could we have met in Malta in 1999?"
"Yes indeed," the answer should be. "I was in La Valetta, but in 2000."
According to Moscow's instructions, the stranger would then slip the spy a false Irish passport, for travel on to Russia.