LONDON—The U.K. government said it has evidence that Russia has spent the past decade secretly developing nerve agents to use in assassinations, in what it said was a violation of international treaties banning chemical weapons.

Foreign Secretary Boris Johnson said Russia has been producing and stockpiling Novichok “within the last decade” as part of a program to develop ways of using nerve agents in assassinations, in a violation of the Chemical Weapons Convention. He didn’t elaborate on where the information came from.

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