Russian diplomats and security chiefs denied Thursday sending their spies to the United States to purchase high-tech military electronics and detonators in faked civilian deals.

The United State on Wednesday disclosed the details about an 11-member spy ring from ex-Soviet nations that allegedly used doctored documents to procure pieces of equipment too advanced for the Russian state.

The arrests were a feather in the cap of the US spy agency that irritated the Russians and notched another rung lower in relations that have suffered since before Vladimir Putin's return to an historic third term as president in May.

Russia has already halted the work of a top US State Department agency responsible for political outreach for its work on behalf of those who organised record rallies against Putin's return.

Putin has also pushed through a law branding political organisations that receive funds from other countries with the loaded term "foreign agents" that to most Russians implies the word spy.

But officials in Moscow were keen Thursday to stress that what they had on their hands this week was not a new spy row but a basic criminal case whose details the Russians were still trying to grasp.

"The charges shown to us by the US side are of a criminal nature and do not involve possible intelligence work," said Deputy Foreign Minister Sergei Ryabkov.

His comments were echoed a few hours later by the powerful director of the FSB security services that inherited most of the functions of the Soviet-era KGB.

FSB boss and close Putin ally Alexander Bortnikov noted all he could say with any degree of certainty was that these seemed to be ex-Russian nationals who were now living somewhere abroad.

But his tongue appeared to have slipped briefly because he also became the lone Russian official to -- incorrectly -- say that the group had been charged with espionage work.

US authorities filed criminal charges to get the men into detention before pressing on with what appears to be a complicated and multi-layered case.

"I issued orders to services responsible for that sort of thing to start gathering information about those detained in the United States for espionage," the state RIA Novosti news agency quoted Bortnikov as saying.

"It would be good to get to the bottom of what really happened," he remarked.

What the US authorities suspect in public is that the group faked papers to ship out parts and pieces the United States would dearly love to keep away from a country building new nuclear reactors for Iran.

The entire operation was allegedly run between 2008 and the present by a Texas-based company called Arc Electronics.

Its Russian-US defendant Alexander Fishenko is being charged under military export laws and accused of being an "unregistered agent of the Russian government."

The traditional note of anger was still present in Russian statements Thursday that questioned why Moscow appeared to have been one of the last to hear about the case.

"Serious questions arise over the fact that the US authorities did not notify Russia's consulate offices about... our compatriots' arrest," foreign ministry spokesman Alexander Lukashevich told reporters.

And Ryabkov raised "serious concerns" because "much in this story is not clear."

Russia's recent anger at Washington has spilled across US party politics and now also covers the presidential campaign of the more Kremlin-cautious Mitt Romney.

The Republican Party hopeful has recently vowed to show "more backbone" with Moscow after calling Russia his country's top "geopolitical foe."

Putin last month dismissed Romney's remarks as posturing commonly assumed during close election campaigns.