The United Kingdom has confirmed that one of its Royal Navy nuclear-powered submarines surfaced close to the Yantar, a notorious Russian spy ship late last year, to make it clear it was being observed.

The UK authorized the British submarine to surface closely to the Russian vessel to warn it off, British defense secretary Healey said.

The Royal Navy of the United Kingdom has been closely tracking a Russian spy ship recently spotted in British waters, sparking concerns about Russia’s activities near the U.K.’s critical underwater infrastructure.