Members of a Russian spy ring are seen in this courtroom sketch during their appearance in Manhattan Federal Court in New York on Thursday. (Christine Cornell/Reuters)

The United States and Russia completed what is believed to be the largest spy swap since the Cold War, exchanging 10 spies arrested in the U.S. for four convicted in Russia on Friday.

After not commenting for days, the U.S. Justice Department in Washington finally announced a successful completion to the spy swap after two planes involved touched down in Moscow and London.

The U.S. and Russian flights carrying the candidates for the 14-person spy swap landed briefly in Vienna earlier Friday, exchanged agents, then took off again.

The 10 Russian agents who infiltrated suburban America and acted as spies for Moscow were deported Thursday after admitting their crimes.

Moscow avoided having 10 spy trials in the United States that would have spilled embarrassing details of how its agents, posing as ordinary citizens, apparently uncovered little of value but managed to be watched by the FBI for years.

The spies left New York hours after pleading guilty to conspiracy in a Manhattan courtroom and being sentenced to time served and ordered out of the country.

They were exchanged for four people convicted of betraying Moscow to the West.

Both countries won admissions of crimes from the subjects of the exchange — guilty pleas in the U.S. and signed confessions in Russia.

In exchange for 10 agents, the U.S. won freedom for and access to two former Russian intelligence colonels who had been convicted in their home country of compromising dozens of valuable Soviet-era and Russian agents operating in the West. Two others also convicted of betraying Moscow were wrapped into the deal.

One ex-colonel, Alexander Zaporozhsky, may have exposed information leading to the capture of Robert Hanssen and Aldrich Ames, two of the most damaging spies ever caught in the U.S.

The largest known Cold War spy swap was in 1985 when 29 people were exchanged on the Glienecke Bridge between East Germany and West Berlin. Four people indicted for spying in the U.S. were exchanged for five Polish prisoners and 20 other alleged spies held in East Germany and Poland.

Russian agents under watch

U.S. officials said several of the four freed by Russia were ailing, and cited humanitarian concerns in part for arranging the swap in such a hurry. They said there was no substantial national security benefit to keeping the captured agents in prison for years. Former intelligence operatives agreed.

"We wanted to make sure that we did this as quickly as we could so we didn't have any kind of ongoing negative impact between the good relationship that is developing between the United States and Russia," said U.S. Attorney General Eric Holder.

The 10 Russian agents arrested in the U.S. had tried to blend into American suburbia but had been under watch for up to a decade by the FBI. Their access to top U.S. national security secrets appeared spotty at best, although the extent of what they knew and passed on is not publicly known.

In an elaborate round of dealmaking, U.S. officials met in Russia on Monday with the convicted spies and offered them a chance for freedom if they left their country. Russian officials in the U.S. held similar meetings with the agents captured by the FBI.

'Sends a powerful signal'

On Thursday, Russian President Dmitry Medvedev signed a decree pardoning the four after officials obtained their confessions.

The Kremlin identified the four as Zaporozhsky, Gennady Vasilenko, Sergei Skripal and Igor Sutyagin. Among them, Sutyagin, an arms control researcher who asserts his innocence despite the confession, was thought to have been flown to Vienna.

"This sends a powerful signal to people who co-operate with us that we will stay loyal to you," said former CIA officer Peter Earnest. "Even if you have been in jail for years, we will not forget you."

He said the U.S. gave up 10 "fairly lightweight operatives" and "in return, we are getting four people who were actually convicted of spying for the U.S. and Britain, by Russia."

The 10 suburbanites pleaded guilty in a Manhattan courtroom Thursday to conspiracy to act as an unregistered agent of a foreign country. An 11th defendant is still a fugitive after he jumped bail in Cyprus.

The defendants, led into court in handcuffs, provided almost no information about what kind of spying they actually did for Russia.

Asked to describe their crimes, each acknowledged having worked for Russia secretly, sometimes under an assumed identity, without registering as a foreign agent.