According to senior administration officials and others briefed on the material, the US has obtained intelligence about a Russian plan to use a faked video to create a pretext for an invasion of Ukraine, building on recent disinformation campaigns.

The plan, which the US hopes to thwart by making it public, entails staging and filming a fabricated attack by Ukrainian forces on Russian territory or against Russian-speaking people in eastern Ukraine.

Russia intended to use the video to accuse Ukraine of genocide against Russian-speaking people, according to officials. It would then use the outrage over the video to justify an attack, or it would have separatist leaders in eastern Ukraine’s Donbas region invite Russian intervention.

Officials refused to release any direct evidence of the Russian plan or how they discovered it, claiming that doing so would jeopardize their sources and methods. However, a recent Russian disinformation campaign centered on false accusations of genocide, as well as recent political actions in the Russian parliament to recognize breakaway governments in Ukraine, lent credence to the intelligence.

The Russian operation, if carried out, would be an extension of a propaganda theme that American intelligence officials and outside experts have said Moscow has been pushing on social media, conspiracy sites, and state-controlled media since November.

Officials said the video was meant to be elaborate, with plans for graphic images of the staged, corpse-strewn aftermath of an explosion and footage of destroyed locations. They claimed that the video would also feature phony Ukrainian military equipment, Turkish-made drones, and actors portraying Russian-speaking mourners.

Officials in the United States would not say who was planning the operation in Russia, but a senior administration official said Russian intelligence was “intimately involved” in the effort.

According to a British government official, they had conducted their own intelligence analysis and were confident that Russia was planning to engineer a pretext to blame Ukraine for an attack. The intelligence details, according to the official, are “credible and extremely concerning.”

While it is unclear whether senior Russian officials approved the operation, officials said it was well underway and the US had high confidence that it was being seriously considered. Russian officials had found corpses to use in the video, discussed actors to play mourners, and planned how to make military equipment appear Ukrainian or NATO-supplied in the video.

While the plan sounded far-fetched, American officials said they believed it could have worked to spark a Russian military operation, which they hoped would be made less likely by publicizing the effort.

The intelligence highlights have been declassified in the hopes of both derailing the plot and convincing allies of the seriousness of Russia’s plans. The officials interviewed for this article asked to remain anonymous in order to discuss declassified but sensitive intelligence before it was made public.

The director of national intelligence, Avril D. Haines, and other top administration officials briefed members of Congress on the information on Thursday. Details of the information have also been shared with allies, as the US and the UK pursue an intelligence diplomacy.

Both Washington and London have outlined elements of Moscow’s war planning in recent weeks, highlighting planned troop buildups, exposing a false-flag sabotage plot, and revealing Russian plans to install a friendly government in Kiev. The strategy seeks to persuade allies that Russia is not bluffing and has real war plans that could be implemented. The releases also aim to force Russia to abandon and rewrite plans, further postponing any invasion plan.

According to diplomats, the longer the international community can postpone President Vladimir V. Putin’s decision on whether to authorize a military operation against Ukraine, the more likely he will reconsider his plans.

The oppression of Russian speakers in eastern Ukraine is a recurring theme in Russian state media and on websites controlled by Russian intelligence services. But, contrary to what Moscow claims, language is not a sharp dividing line in Ukraine.