The conflict in Ukraine's Donbas has been going on for eight years
Russia is planning to fabricate a pretext for an invasion of Ukraine, by falsely blaming Ukrainian military for an attack on Russian-backed separatists or Russia itself, US officials say.
One option Russia is said to be considering is to stage and film a fake attack, with graphic images of an explosion showing numerous casualties.
In response, Russia said it was not planning any false flag operations.
The US and Nato are concerned at the massing of Russian forces near Ukraine.
Russia denies planning to invade, saying the troops are there for drills. They currently number about 100,000.
The tensions come eight years after Russia annexed Ukraine's southern Crimea peninsula and backed a bloody rebellion in the eastern Donbas region.
Senior US administration officials said the alleged false flag operation, planned by Russian security services, would show images of civilian casualties in the Donbas, in order to generate outrage against the Ukrainian authorities.
This could then be used to justify an attack on Ukraine, the officials said. though they did not release any evidence to support their claims.
The plan could involve staging and filming a faked attack, they added.
It would show corpses and destroyed locations, faked Ukrainian military equipment, Turkish-made drones and actors playing Russian-speaking mourners, they said.
But the officials stressed that this was only one of the options Russia was considering, and said they were publicising it in an effort to "dissuade Russia from its intended course of action".
UK Foreign Secretary Liz Truss said the US intelligence "is clear and shocking evidence of Russia's unprovoked aggression and underhand activity to destabilise Ukraine".
"This bellicose intent towards a sovereign, democratic country is completely unacceptable and we condemn it in the strongest possible terms. The UK and our allies will continue to expose Russian subterfuge and propaganda and call it out for what it is," she said in a statement.
Kremlin spokesman Dmitry Peskov responded to the reports later on Thursday.
"This is not the first promise of its kind [to release details about Russian provocation]," he said, quoted by Tass news agency. "Something similar was also said before, but nothing came of it."
Meanwhile, Russian Ambassador to the EU, Vladimir Chizhov, told CNN that Moscow was not planning any false flag operations to invade Ukraine.
News of the alleged plot came a day after the US said it was sending more troops to Nato allies in Europe.
Russia said the move was "destructive" and showed that its concerns about Nato's eastward expansion were justified.
Also on Thursday, the Nato military alliance expressed concerns that Russia was likely to deploy up to 30,000 troops - including special forces, fighter jets and short-range ballistic missiles - in Belarus, Ukraine's northern neighbour.
"This is the biggest Russian deployment there since the Cold War," Nato Secretary General Jens Stoltenberg said.
Rivalry between Russia and the US, which still possess the world's biggest nuclear arsenals, dates back to the Cold War (1947-89). Ukraine was then a crucial part of the communist Soviet Union.
Intense diplomacy continued on Thursday to try to prevent what a number of military experts fear could be a major war in Europe.
During his visit to Ukraine's capital Kyiv, Turkish President Recep Tayyip Erdogan again offered to broker talks between Ukraine and Russia.
"Turkey is ready to do its part to resolve the crisis," Mr Erdogan said. Turkey has good relations both with Ukraine and Russia.
Meanwhile, French President Emmanuel Macron held telephone talks both with Russian President Vladimir Putin and his Ukrainian counterpart Volodymyr Zelensky to try to defuse tensions.
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