Russia announced 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 F.SB., said that the decision had been made in response to “the numerous unfriendly steps taken by London,” a possible 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 that 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.”

On Friday, the Kremlin’s spokesman, Dmitri S. Peskov, told reporters that Mr. Putin’s statement was “very important” and that the Russian government had “no doubt” that the message “had reached its addressees.”

The British Foreign Office — which said the accusations of spying by the diplomats were baseless — said that the six had left Russia last month, after Moscow’s notification to London about its decision in early August.

That came after Britain said in May that it was expelling a Russian defense attaché as an undeclared military intelligence officer and that it was imposing other restrictions on the Russian Embassy in London.