Russia's FSB security service said Thursday it opened a criminal case against a US citizen suspected of espionage, as ties between Moscow and Washington disintegrate further over the Kremlin's nearly year-long Ukraine offensive.

"The Federal Security Service of the Russian Federation initiated a criminal case against a US citizen on the grounds of a crime under the 276 'Espionage' Article of the Criminal Code," the FSB said.

"The American is suspected of collecting intelligence information in the biological sphere, directed against the security of the Russian Federation," it added.

It gave no further details.

Ties between Russia and the United States, already strained for years, have reached new lows since President Vladimir Putin sent troops to Ukraine last year.

Several Americans are currently in detention in Russia and both countries accuse each other of political arrests.

There have been several high-profile prisoner exchanges between Moscow and Washington.

Last month, Moscow freed US basketball start Brittney Griner -- arrested for bringing cannabis oil into the country -- in exchange for Russian arms dealer Viktor Bout.

Paul Whelan, a former US marine, was arrested in Russia in 2018 and given a 16-year sentence on "espionage charges". He remains in prison.