Since the start of his full-scale war, President Vladimir V. Putin of Russia has tried to drive wedges between Ukraine and its Western backers, warning of possible grave consequences as they supplied Kyiv with money and weapons. On Thursday, the British government suggested that Russia had put a new spin on that effort, as Moscow accused Ukraine of planning a false-flag attack on Britain’s ambassador to Russia.

Russia’s Federal Security Service, known as the F. S. B., arrested an unidentified Russian man, described as a former security guard at the British Embassy in Moscow, and accused him of treason.

In elaborate and convoluted accusations, laid out in videos and a lengthy statement, the F. S. B. said the security guard was feeding information to Ukrainian intelligence so that Kyiv could attack the ambassador and other British diplomats and blame Russia.

Britain, a staunch backer of Ukraine, declined to comment on the case, beyond saying it was common for Russia to seek to sow divisions between Ukraine and its international partners through disinformation. Kyiv warned that Russia may be planning such acts itself. "Moscow is trying to shift responsibility in advance for sabotage or other actions aimed at escalation that Russia itself may be preparing against Great Britain," Ukraine’s Center for Countering Disinformation, a government agency, said in a statement. The center called such tactics "a typical tool of Russian propaganda and special services." In one F.

S. B. video released on Thursday, plainclothes officers in plaid flannels pinned down the man described as the former security guard, who later appeared on camera with a blurred face and gave a stilted confession. Speaking in sentences that sounded memorized, the man claimed to have been spying on British diplomats and providing the information to Dmytro Karpenko, a well-known Ukrainian blogger who interviews Russian prisoners of war in videos he posts online.

The man said the information was being gathered so the Ukrainian government could carry out acts of sabotage against the British ambassador, as well as against his residence and the embassy. In a statement, the F. S.

B. said that Ukraine was planning to pin the attacks on Moscow to draw Britain into a direct armed conflict with Russia, "ensure additional supplies of weapons and military equipment from London to Ukraine, and increase sanctions pressure on Russia." Mr. Karpenko did not immediately respond to a request for comment. Russia released a second video of an anonymous person in a suit who was described as an "F. S.

B. operative." The person was shown only from the back, speaking under a dark shadow beside a lit-up red-and-silver emblem of the agency. The "F. S.

B. operative" suggested that British intelligence should have expected such "betrayal and a knife in the back" from the Ukrainians, having "created this homunculus in its own image and likeness." A homunculus is a little man. The Russian security service also warned Russian citizens against speaking with Mr. Karpenko, who goes by the name Apostle and at times calls the relatives of Russian prisoners of war during filming. Russia’s vitriol over Western support for Ukraine was once directed primarily at the United States.

But since President Trump returned to the White House, began peace talks and dialed down American support for Ukraine, Russian propaganda broadsides have focused instead primarily on Britain and other European nations. On the battlefield, Britain remains one of Ukraine’s most powerful and steadfast supporters, aggressively helping Kyiv with weapons and intelligence in the fifth year of Russia’s full-scale war. Mr. Putin has sought to peel off Western support for Ukraine with fear tactics since the day he announced the invasion in February 2022. In an early-morning address that day, the Russian leader obliquely threatened to use nuclear weapons against any nation that intervened on Ukraine’s behalf, telling those countries that they would face "such...