The UK government said Sunday it has evidence that Russia has spent the last decade secretly developing nerve agents to use in assassinations, in what it said was a violation of international treaties banning chemical weapons.

“We have evidence that Russia has been investigating delivery of nerve agents and has been creating and stockpiling Novichok,” Foreign Secretary Boris Johnson told the British Broadcasting Corp., referring to the poison British authorities say was used in the attempted murder of Sergei Skripal, a Russian colonel who spied for the British and was traded in a spy swap in 2010, according to the Wall Street Journal.

His statement marks the latest escalation in a diplomatic crisis between London and Moscow.  Mr. Skripal and his daughter Yulia remain gravely ill after being found slumped on a park bench in the English city of Salisbury earlier this month. The U.S., France and Germany have joined the UK in blaming Moscow for the attack, which also injured a police officer.

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In a statement, Britain’s Foreign Office said its information indicates that 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. It didn’t elaborate on the nature of its intelligence or where it came from.

Russia denies it was behind the attempted killing of Mr. Skripal.