Britain said Wednesday that two officials from Russia's GRU intelligence agency carried out a poisoning attack on a former spy on British soil in March, using a Nina Ricci perfume bottle containing a nerve agent.

- What is the GRU? -

GRU stands for the Main Intelligence Directorate, Russia's military intelligence agency which is one of Moscow's three spy agencies along with the FSB security service and the SVR foreign intelligence agency.

Still commonly known as GRU, the agency changed its name to the Main Directorate (GU) as a result of a reform in 2010 .

Its chief Igor Korobov reports to the Chief of the General Staff Valery Gerasimov and Defence Minister Sergei Shoigu.

The agency's structure, number of employees and finances constitute a state secret. Its official emblem is a black bat flying above the globe.

- What does it do? -

It has an extensive spy network abroad and its highly-trained "spetsnaz" special forces have fought in various conflicts, including in Afghanistan and Chechnya.

In recent years the agency has gained notoriety for its connection to some of Russia's most contentious actions abroad.

In July this year, US Special Counsel Robert Mueller indicted 12 GRU officers, accusing them of interfering in the 2016 US presidential election.

The Bellingcat investigative team has linked the downing of MH17 in eastern Ukraine in 2014 with a GRU officer who it said "supervised the procurement and transport of weapons."

The agency has also been linked to an attempt to overthrow Montenegro's government on the eve of parliamentary elections in October, 2016.

GRU officers are believed to provide military guidance to the Syrian army as well as rebels in eastern Ukraine, among others.

- Does it kill traitors? -

In 2010, Putin famously said that Russian special services do not kill traitors, saying they "croak all by themselves."

He acknowledged that Soviet authorities deployed assassins to get rid of enemies, but said modern Russia no longer does so.

"Such units have been liquidated," he said.

Earlier this year, a former GRU chief, Fyodor Ladygin, speaking to the defence ministry's TV channel Zvezda, denied his agency's involvement.

"The Russian intelligence structure where I had the honour of serving for many years...never -- I repeat never -- did such vile, stupid things," he said.

- Analysts' take -

Military analysts were hard-pressed to say whether the Main Intelligence Directorate could be behind the assassination attempt in the British cathedral city of Salisbury in March, when Russian former double agent Sergei Skripal and his daughter Yulia were poisoned.

"I never heard about GRU being involved in political murders," independent security analyst Alexander Golts told AFP.

He suggested that "theoretically" someone could have acted on his own initiative to obtain promotion.

Another prominent military analyst,Pavel Felgenhauer, suggested the murder attempt with the use of Novichok, a military-grade nerve agent developed by the Soviet Union, was a collective effort involving several spy agencies.

"Such a decision would have been impossible without permission from the Kremlin," he told AFP.

"The goal of such an operation is to send a message to your own people: no mercy for the traitors and we don't care about the consequences for the country's image."

British police accuse GRU officers, identified as Alexander Petrov and Ruslan Boshirov, of trying to kill Skripal and his daughter Yulia.

Skripal, a former colonel in Russian military intelligence, was jailed for betraying agents to Britain's MI6 security service. He moved to England in 2010 as part of a spy swap.

Several top GRU agents have defected to the West in the past.

Vladimir Rezun, who defected in the 1970s, wrote a partly autobiographical book under the penname Viktor Suvorov. The book is entitled Aquarium, the nickname of the GRU headquarters in Moscow.

"What kind of fish swim there?" he asks his boss in the book. "There's only one kind there -- piranhas."