Ten alleged members of a Russian spy-ring have been charged in the US with acting as foreign agents.
The suspects are accused of posing as ordinary citizens, some living together as couples for years.
They were charged with conspiracy to act as unlawful agents of a foreign government, a crime which carries up to five years in prison.
A Russian foreign ministry spokesman said the allegations were contradictory.
"We are studying the information. There are a lot of contradictions," spokesman Igor Lyakin-Frolov told the AFP news agency, declining further comment.
Russian Foreign Minister Sergei Lavrov later said Moscow expected Washington to provide an explanation over the the spying row, Russia's Interfax news agency reports.
Nine of the alleged spies also face a charge of conspiracy to launder money, which carries a 20-year prison sentence.
An 11th suspect remains at large, according to the US justice department.
Alleged intercepted messages in court documents suggest they were asked to find information on topics including nuclear weapons, US arms control positions, Iran, White House rumours, CIA leadership turnover, and political parties.
The US Department of Justice says eight of the suspects allegedly carried out "long-term, 'deep-cover' assignments" on US soil, working in civilian jobs so as not to arouse suspicion.
They were allegedly trained by the Russian Foreign Intelligence Service (SVR) to infiltrate policy-making circles and collect information, according to court papers filed in the US court for the southern district of New York.
They were told to befriend US officials and send information using various methods to Russian government handlers.
US officials said the spy-ring was discovered in a "multi-year investigation" by FBI agents who posed as Russian handlers and gleaned information from two of the suspects.
Investigators say some of the agents had been using false identities since the early 1990s, using codes and engaging in advanced computer operations, including posting apparently innocent pictures on the internet which contained hidden text.
The FBI also reported observing older techniques, such as messages sent by invisible ink, money being buried next to a beer-bottle marker and "brush pasts" in parks, where agents swap identical bags as they pass each other.
"You were sent to USA for long-term service trip," says one purported message to two of the suspects that was intercepted by US intelligence.
"Your education, bank accounts, car, house etc - all these serve one goal: fulfil your main mission, ie to search and develop ties in policymaking circles in US and send intels."
Generally, spies were allegedly tasked with becoming "Americanised" to be able to do this, with some pursuing university degrees, holding jobs, and joining relevant professional associations, court documents said.
The group allegedly got close to a scientist involved in designing bunker-busting bombs and a top former intelligence official.
Five of the suspects briefly appeared in a Manhattan federal court on Monday, where a judge ordered them to remain in prison until a preliminary hearing set for 27 July.
These included a couple known as "Richard Murphy" and "Cynthia Murphy", who were arrested in Montclair, New Jersey; Vicky Pelaez and a man known as "Juan Lazaro," who were arrested in Yonkers, New York state; and Anna Chapman, who was arrested in Manhattan, New York City.
Another three - Mikhail Semenko and a couple known as "Michael Zottoli" and "Patricia Mills" - appeared in a federal court in Alexandria, Virginia, after being arrested in Arlington, Virginia.
The final two people - a couple known as "Donald Howard Heathfield" and "Tracey Lee Ann Foley" - were arrested in Boston, Massachusetts, and appeared in a federal court in the city.
A suspect known as "Christopher R Metsos" remains at large.
All the suspects except Ms Chapman and Mr Semenko have also been charged with conspiracy to commit money laundering.
Relations between Washington and Moscow have warmed in recent months.
Last week, Russian President Dmitry Medvedev was in Washington having lunch with President Barack Obama.
A senior government official told the BBC that it was unfortunate that such activity was taking place in the US, but that it should not affect the momentum established in the relationship with Russia.