VIENNA: In a perfectly choreographed operation, Russia and the United States completed their biggest spy swap since the Cold War Friday, exchanging 10 agents deported by the US for four freed by Moscow.

As the cloak-and-dagger move successfully drew the curtain on what could have fuelled an embarrassing diplomatic spat between the two nations, more details began to emerge highlighting the high-stakes involved.

Special Russian and US flights carried the spies to Vienna early Friday, parked next to each other on the runway, then took off within 15 minutes of each other after the exchange, which was kept well hidden from banks of media.

A government jet flew the 10 Russian spies, including the glamorous Anna Chapman, back to Moscow`s Domodedovo airport, officials said.

Pictures broadcast on Russian state television showed the agents being whisked away from the airport tarmac to an unknown location in two minivans.

The American plane, meanwhile, made a brief stop at the Brize Norton air base in central England, British media reported, before taking off again and landing around 2130 GMT at Dulles international airport outside Washington.

It was not immediately clear if all the four agents released by Russia were brought to the United States, as US and British media reported one, and maybe two, of them had stayed behind in Britain.

The White House meanwhile revealed it had been first briefed about the "deep cover" Russian spy network as early as February, well before the Russian agents were arrested in an FBI swoop on June 27 after a decade-long surveillance.

And it confirmed that CIA chief Leon Panetta had led negotiations with the Russians on resolving the crisis, which flared into the open just as the former Cold War foes are enjoying a "reset" in relations.

Vienna, the Austrian capital near the old Iron Curtain frontier, has not seen such drama since the Cold War, when it was at the heart of the espionage struggle played out between the two superpowers.

"The United States government came up with the four individuals to be freed by the Russians based on humanitarian concerns, health concerns, and other reasons that we put forward to the Russians," the White House said in a statement, revealing how the deal was hammered out.

The Russian foreign ministry said the swap involved the "return to Russia of 10 Russian citizens accused in the United States, along with the simultaneous transfer to the United States of four individuals previously condemned in Russia."

The United States sent back the 10 Kremlin agents late Thursday after they pleaded guilty in a New York court to acting as illegal agents. They were immediately expelled.

The four released by Russia included Igor Sutyagin, convicted in 2004 of handing over classified information to a British company that Russia claimed was a CIA cover. He was serving a 15-year jail term.

Russian President Dmitry Medvedev pardoned the four on Thursday after they signed documents admitting they had spied.

Alongside Sutyagin, the four released by Russia included Sergei Skripal, a former colonel with Russian military intelligence; ex-Russian Foreign Intelligence agent Alexander Zaporozhsky; and Gennady Vasilenko about whom far less is known.

Former homeland security advisor Fran Townsend told CNN Friday that those agents who had returned to the United States now faced a long debriefing in a CIA safe house, which could take weeks or months.

The debriefings will help "from a counterintelligence perspective," she said, adding: "We will likely learn a fair amount from these people."

The CIA would also help them with new identities and adapting to a new life, providing housing and financial assistance, she said.

Despite the diplomatic storm caused by the spy ring, the group appeared to have been amateurish and made little impact in the decade since being formed.

In court, several defendants acknowledged using fake names. The couple living as Richard and Cynthia Murphy were really Vladimir and Lydia Guryev, while Donald Heathfield`s true name was revealed to be Andrey Bezrukov.

BDST:0921 HRS , July 10, 2010