LONDON, April 13 (Reuters) - Russia's intelligence agencies have been spying on former double agent Sergei Skripal and his daughter Yulia for at least five years, Britain's National Security Adviser Mark Sedwill has told the North Atlantic Treaty Organisation, according to a Press Assocation tweet on Friday.

The comments came in a letter from Sedwill to NATO Secretary General Jens Stoltenberg, the PA said.

Officials at NATO were not immediately available to comment. (Reporting by Kate Holton; writing by Elisabeth O'Leary; editing by Stephen Addison)

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