Image caption The SVR operates from a headquarters outside Moscow

Russia's External Intelligence Service (SVR) is the current incarnation of one of the world's oldest and most extensive espionage agencies, known for decades as the KGB.

It officially celebrates its 90th birthday in 2010, tracing its lineage back to the Soviet Union's NKVD Foreign Department, set up on 20 December 1920.

The KGB (Committee of State Security) moniker surfaced in the 1950s, when it was officially known as the KGB's First Main Directorate, to distinguish it from the domestic secret police.

The SVR's closeness to the Kremlin is underlined by the fact that its current director, Mikhail Fradkov, and one of his predecessors, Yevgeny Primakov, both served as prime ministers of Russia.

Even in the worst years of the Cold War, I think there were no more than 10 illegals [deep-cover Soviet spies] in the US, probably fewer Oleg Kalugin, former KGB general

But it is the current Russian Prime Minister, Vladimir Putin, who is undoubtedly the service's best-known graduate, having served as an agent in East Germany in the 1980s.

Today the SVR describes itself as a "modern special service employing talented, ambitious people devoted to the Motherland and their military duty".

According to a recent German government report on the service and its latter-day German operations - a report which caused indignation in Moscow - the SVR currently has 13,000 employees.

If allegations are proven that the SVR built up an extensive ring of spies in the US in the first decade of the 21st Century, it would be only the latest in a line of such operations, stretching back to at least the 1940s when the famous Soviet spy Rudolf Abel went operational in Brooklyn.

New opportunities

The length of time that the alleged New York ring is supposed to have been active is interesting, the BBC's James Rodgers writes.

Image caption The US arrested 10 suspected spies

If it has been working since 2000, it started at a time when relations between Russia and the US were still, to some extent, enjoying the warmth which came with the end of the Cold War.

This was before issues such as Nato's eastward expansion,Russia's war with Georgia and missile defence put a strain on ties between the two countries.

Of course, there are plenty of examples of politicians doing one thing while spies do another, our Europe regional editor says, but perhaps the Russian intelligence services even then anticipated tensions to come?

Perhaps, also, in the Russia of the 1990s, they were pretty free to pursue their own agenda?

Our editor notes that the post-Cold War improvement in relations between Washington and Moscow also seems to have facilitated the setting up of a spy ring by making it easier for Russians to move to the US.

Anyone who was in Moscow in the early 1990s, he recalls, will remember the massive influx of business people, consultants, aid workers and others from the West, and presumably there were a few intelligence agents among them, too.

The 'illegals'

The New York spies, if that is what indeed they are, would be considered "illegals" in espionage jargon, meaning they operated without diplomatic or other "cover".

THE SVR Was known as KGB until 1991

Based outside Moscow, said to have staff of 13,000

Works in parallel with Russian military intelligence (GRU)

Has its own academy

Col Rudolf Abel, or William Fischer (Russian: Vilyam Fisher) to give him his real name, was an "illegal" par excellence.

He was sent to the US in either 1948 or 1949 to gather information about America's burgeoning nuclear programme, the SVR records on its website.

Born and raised in England, the son of Russo-German exiles, he passed himself off as a native-born New Yorker named Emil Goldfus, a photographer by trade. Intelligence contacts knew him simply as Mark.

His covert work was to last nearly a decade, during which time he was tasked with reorganising the whole "illegal" network in the US and setting up his own system of radio communications with Moscow.

Unmasked by the FBI in 1957, he was tried and imprisoned before being famously swapped in Berlin for captured US spy plane pilot Gary Powers in 1962.

Another famous "illegal" was Konan Molody, who entered the UK in 1954 as a Canadian businessman by the name of Gordon Arnold Lonsdale, and organised a ring to spy on the British submarine detection programme.

Caught and arrested in 1961, he too was freed in a swap.

Oleg Kalugin, a former KGB general now living in self-imposed exile in the US, told the New York Times he was shocked at the large scale of the alleged New York operation.

"It's a return to the old days, but even in the worst years of the Cold War, I think there were no more than 10 illegals in the US, probably fewer," he told the newspaper.