What is Russia's GRU military intelligence agency?

Britain said two Russian military intelligence officers, almost certainly acting with the approval of senior Russian officials, were responsible for the attempted murder of former Russian spy Sergei Skripal and his daughter with a nerve agent.

What is the GRU?

* Russia's military intelligence service is commonly known by the Russian acronym GRU, which stands for the Main Intelligence Directorate. Its name was formally changed in 2010 to the Main Directorate (or just GU) of the Russian chief of the general staff, but its old acronym - GRU - is still more widely used.

The GRU was founded as the Registration Directorate in 1918 after the Bolshevik Revolution. Revolutionary leader Vladimir Lenin insisted on its independence from other secret services, which saw it as a rival.

Russia's two other most widely known intelligence and security services were both created from the Soviet-era KGB: the Foreign Intelligence Service, or SVR, and the Federal Security Service, or FSB.

* The GRU answers directly to the chief of the general staff and the Russian defense minister, each of whom is thought to have access to Russia's portable nuclear briefcase.

Igor Korobov is the chief of GRU. He was sanctioned by the U.S. Treasury in 2016 and this year accused of attempted interference in the U.S. election and cyber attacks.

Valery Gerasimov is currently chief of the general staff. He was sanctioned by the EU over Russia's annexation of Ukraine.

The GRU does not have a website and does not comment publicly on its actions. Its structure, staff numbers, and financing are state secrets.

* The GRU has agents across the globe. It also has special forces units that fought in many post-World War Two conflicts including Afghanistan and Chechnya.

* The public was given a rare chance to see parts of the GRU's Moscow headquarters when President Vladimir Putin visited it in 2006. He was shown taking part in shooting practice.

* Sergei Skripal once worked in the GRU but was turned by Britain's MI6 spy service. He is believed to have betrayed dozens of GRU spies across Europe to his British handlers.