Russia said on Friday that it had decided to expel six British diplomats from the country, accusing them of engaging in espionage and sabotage work, in a move that highlighted the deepening tensions between Moscow and London.

The Russian Federal Security Service, or FSB, said that the decision had been made in response to “the numerous unfriendly steps taken by London”, an apparent reference to signals from Britain that it was willing to allow Ukraine to use its “Storm Shadow” long-range missiles against targets deep inside Russia.

Speaking on Thursday about the potential shift on the use of missiles, President Vladimir V. Putin warned that such a decision would mean that NATO countries were “at war with Russia” and that it would “clearly change the very essence, the very nature of the conflict.”

President Joe Biden and his western allies were still deliberating the potential benefits and risks of allowing Ukraine to use long-range missiles as Prime Minister Keir Starmer traveled to Washington for his first official visit as the British leader.

The FSB, the main successor agency to the Soviet-era KGB, said that the six expelled diplomats had been sent to Russia by the British foreign service directorate responsible for Eastern Europe and Central Asia.

Since the invasion of Ukraine, the agency’s statement said, the British directorate has turned into “a special service, whose main task is to inflict strategic defeat” on Russia.

The FSB said that the activities of the six diplomats were “threatening” to Russia’s security and that “signs of spying and sabotage” had been detected in their work.

“The revealed facts give grounds to consider the activities of British diplomats sent to Moscow through the directorate as threatening the security of the Russian Federation,” the FSB said.

The British foreign office rejected the accusations of spying by the six who were expelled, describing the claims as “baseless”.

Maria V. Zakharova, a spokeswoman for the Russian foreign ministry, said in a statement that the ministry supported the FSB’s decision.

She said that the six British diplomats had been engaged in “subversive work aimed at harming” Russian people. In a separate statement, the Foreign Ministry promised to expel more British diplomats should they engage in similar activities.

New York Times News Service