Ten people accused of spying for Russia have tonight pleaded guilty to a conspiracy charge, setting up what could be one of the biggest, most unusual and least secret spy swaps known to superpower espionage.

The defendants, members of a deep-cover spy ring broken up last week, all told the federal judge in New York they would plead guilty.

They were charged with being long term "deep cover" spies, eight of them posing as married couples with children. But they were not accused of collecting classified information.

Two Obama administration sources said tonight that the Russian government would release four alleged western agents.

Igor Sutyagin, a Russian scientist convicted of working for the US, was reportedly flown to Vienna today as a first step toward the release of more than 20 alleged agents held in the US and Russia.

Britain was directly involved in the swap, officials made clear.

Sutyagin, an arms control analyst jailed for 14 years for passing military secrets to a British company the Russian authorities said was a CIA front, was reported to be bound for the UK after his release from a Moscow prison.

Today, riot police secured the perimeter of the former KGB Lefortovo jail in Moscow where Sutyagin was being held, as a convoy of armoured vehicles arrived. A few hours later the Russian media reported he was seen leaving a plane in Vienna, but his family said it was "speculation".

Sutyagin's father, Vyacheslav, said he had received no official confirmation of his son leaving Moscow or arriving in Vienna.

"There have been some unconfirmed reports that Igor flew in to Austria earlier this afternoon, but so far it seems to be wishful thinking. We are waiting for Igor to call us himself. We had expected it to be today, but it looks like it could be tomorrow."

The Russian Gazeta.ru website reported that Anna Chapman, one of the Russian spy suspects arrested in America, might be delivered to Moscow in exchange for Sutyagin.

It quoted a diplomatic saying the 28-year old businesswoman would be flown home in the coming days.

Sutyagin has consistently denied being a spy, saying the information he supplied was available from open sources. But his family said he agreed to effectively be forced in to exile rather than face another four and half years in the "harsh regime" penal colony at Kholmogory near Arkhangelsk. His mother, Svetlana, said he was unshaven and gaunt when she saw him today at Lefortovo.

Moscow was reportedly preparing to release several Russians convicted of working for the CIA or MI6.

Lawyers for the alleged deep cover Russian agents held in the US speculated that they could be on their way to Moscow within hours provided a court approves a deal for them to plead guilty to a single charge of failing to declare payments from a foreign government. They are likely to receive a minimal sentence of the time they have spent in jail since their arrests and then agree to be deported.

The true identities of five of the 10 alleged spies detained in the US are still not known to the US authorities.

Eight were living as married couples with children, some of whom were born in the US. They explained away their accents by claiming to be from Canada or Italy.

The fate of at least two of the accused agents remains in question. Chapman is believed to hold a British passport as well as Russian nationality, while another of the 10 is a US citizen.

An eleventh suspect, Christopher Metsos, who is accused of being the spy ring's paymaster, is on the run after skipping bail in Cyprus.

While their departure may avoid any potential embarrassment to either government that a trial might pose, the alleged spies leave behind them considerable disagreement over how seriously to take their espionage ring.

While the FBI has portrayed the deep-cover "sleeper" agents as a threat to American security, their at times bumbling attempts to infiltrate high policymaking circles has made them figures of fun to many Americans.

Chapman has become such a celebrity that a New York newspaper lamented her departure and asked if the city could keep her.

Yet there is evidence that the Russian intelligence service, the SVR, put considerable effort in to the operation, obtaining false identities using, in one instance, that of a Canadian who died as a child in 1963. The SVR also sent hundreds of thousands of dollars in funds.

The putative swap deal emerged when Sutyagin's lawyer and relatives told the media that prison authorities had abruptly moved him from the penal colony near Arkhangelsk to Lefortovo on Monday.

There is speculation in Russia about which other prisoners jailed for treason and espionage might be part of the swap.

Sutyagin, who is married with two daughters, told his mother he had learned of one other name on the list to be exchanged: Sergei Skripal, a military intelligence officer jailed in Russia in 2006 for giving information to MI6.

A Russian intelligence source told the Kommersant newspaper of two other proposed individuals: Alexander Zaporozhsky, an SVR operative sentenced to 18 years for espionage in 2003, and Alexander Sypachev, jailed for eight years in 2002 for working for the CIA. But Sypachev's lawyer said he would not agree to such a deal.

The putative exchange is particularly interesting as the Russians rarely give up Russians they have jailed on spying charges, well-placed sources said.

One reason given for the extreme reticence of British agencies to talk about the spy swap was the fear the Russians might make fresh arrests to use more people as potential collateral for exchanges. It is possible they were already placing potentially vulnerable people under surveillance now, the sources added.