Ukrainian government websites were hit with a warning today to "be afraid and expect the worst".

Russia has denied involvement, despite having now amassed more than 100,000 troops on its neighbour's borders and releasing pictures of more of its forces on the move earlier today.

A US government official has now said there are concerns "Russia has already pre-positioned a group of operatives to conduct a false-flag operation in eastern Ukraine".

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

"Our information also indicates that Russian influence actors are already starting to fabricate Ukrainian provocations in state and social media to justify a Russian intervention and sow divisions in Ukraine.

"For example, Russian officials and influence actors are emphasising narratives about the deterioration of human rights in Ukraine and the increased militancy of Ukrainian leaders."

White House press secretary Jen Psaki told reporters at her daily briefing that the intelligence shows Russia "is laying the groundwork to have the option of fabricating a pretext for invasion".

Ms Psaki added that Russians appear to be following the same methods they used when they took control of Crimea in 2014.

She said: "We saw this before, leading up to 2014, just to note - through sabotage activities and information operations, by accusing Ukraine of preparing an imminent attack against Russian forces."

She added that an invasion by Russia now "may result in widespread human rights violations and war crimes should diplomacy fail to meet their objectives".

The comments came hours after Ukraine was hit by a massive cyberattack.

Ukraine's foreign ministry spokesperson said it was too early to say who could be behind the attack, but that Russia had been behind similar strikes in the past.

Ms Psaki added President Joe Biden had been briefed on the attack and that the US has "offered our support as Ukraine investigates the impact and recovers from the incidents".

Kremlin spokesperson Dmitry Peskov dismissed the reports as based on "unfounded" information, TASS news agency reported.