U.S. President Donald Trump has imposed another round of sanctions on Russia over the poisoning of a former spy in Britain, the White House said on Friday, a move Moscow said would hurt already strained U.S.-Russia ties.

The move came hours before a landmark Cold War-era arms control treaty expired, after Washington withdrew, accusing Moscow of violating it, which Russia denies.

Washington imposed an initial batch of sanctions last year on Russia after determining that Moscow had used a nerve agent against a former Russian double agent, Sergei Skripal, and his daughter Yulia, in Britain, which Moscow denies.

“After the first round of sanctions in response to Russia’s assassination attempt against a private citizen in the U.K., Russia did not provide the assurances required under U.S. law," said White House spokesman Hogan Gidley.

"So we are imposing a second round of sanctions. It’s another instance where we’re proving tougher on Russia than previous administrations.”

Skripal, a former colonel in Russia’s GRU military intelligence service, and his daughter were found slumped on a bench in the southern English city of Salisbury in March last year after Novichok nerve agent was smeared on his front door.

A woman who lives nearby later died after her partner found the poison in a discarded perfume bottle and brought it home.

European countries and the United States expelled 100 Russian diplomats after the attack. London has charged two Russian men in absentia. The men appeared on Russian TV saying they had visited Salisbury as tourists.