Says Russia as it rejects US allegations that it is preparing stage for invading Ukraine

The United States has accused Russia of sending saboteurs trained in explosives to stage a pretext to invade Ukraine, where government websites were knocked out in a cyberattack linked to Moscow.

The allegations and incident mark a striking new escalation in tensions over Ukraine, just after a week of talks between the West and Russia that sought a diplomatic solution.

Russia has amassed tanks, artillery and tens of thousands of troops near the border of Ukraine as it demands guarantees that its neighbour will never join Nato -- which on Friday announced new cyber cooperation with Kyiv in response to the attack.

Detailing intelligence findings, the White House Friday said that Russia was "laying the groundwork to have the option of fabricating a pretext for invasion" by blaming Ukraine.

"We have information that indicates Russia has already prepositioned a group of operatives to conduct a false-flag operation in eastern Ukraine," said Jen Psaki, the White House press secretary.

"The operatives are trained in urban warfare and in using explosives to carry out acts of sabotage against Russia's own proxy forces."

US intelligence believes Russia could begin the operations several weeks before a military invasion, which could start between mid-January and mid-February, Psaki said.

Russia has denied plans to invade Ukraine and quickly dismissed the latest US statements, with Kremlin spokesman Dmitry Peskov calling them "unfounded".

With the world on alert for any signs of invasion, government sites across Ukraine including of the emergencies ministry, education ministry and cabinet went down early Friday.

Ukraine was still conducting an investigation but preliminary indications suggested that "hacker groups associated with the Russian secret services may stand behind today's massive cyberattack on government websites", foreign ministry spokesman Oleg Nikolenko said on Twitter.

European Union foreign ministers, meeting in the French city of Brest, promised support, with several saying that they had feared a cyberattack to set the stage for a Russian invasion.

US President Joe Biden has warned his counterpart Vladimir Putin in two telephone calls of severe economic consequences if Russia invades.

Moscow said the military drill was a response to what it sees as the growing presence of Nato in its sphere of influence, where it fiercely opposes the expansion of the Atlantic alliance.

Deputy Foreign Minister Sergei Ryabkov said Thursday that Moscow saw no reason to hold a new round of security talks with the West following what he saw as no progress in talks in Geneva, Brussels and Vienna.