The United States said it feared Russia was preparing a pretext to invade Ukraine if diplomacy fails to meet its objectives, after a massive cyber attack splashed Ukrainian government websites with a warning to "be afraid and expect the worst".

Moscow dismissed such warnings, though it has amassed more than 100,000 troops on its neighbour's frontiers and today released pictures of more of its forces on the move.

Russia has also amassed tanks and artillery near the border of Ukraine as it demands guarantees that Ukraine will never join NATO - which today announced new cyber cooperation with Kiev in response to the attack.

Detailing intelligence findings, the White House 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, Ms 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 today.

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 cyber attack on government websites", foreign ministry spokesman Oleg Nikolenko said on Twitter.

The hacked websites read a message in Ukrainian, Russian and Polish: "All information about you has become public, be afraid and expect the worst."

White House Press Secretary Jen Psaki at media briefing today

But Ukraine's SBU security service said access to most sites was restored without hours and preliminary information showed that no personal information was leaked.

NATO said that its experts were on the ground in Ukraine to offer support.

"In the coming days, NATO and Ukraine will sign an agreement on enhanced cyber cooperation, including Ukrainian access to NATO's malware information sharing platform," Secretary General Jens Stoltenberg said.

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

"Some say the cyberattack could be the prelude for other activities, military activities," Austrian Foreign Minister Alexander Schallenberg told reporters.

Russia has repeatedly been accused in hacking attacks in the ex-Soviet country and in the West.

In October 2020, the United States charged six Russians with carrying out cyber attacks on Ukraine's power grid, the 2017 French elections and the 2018 Winter Olympics.

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

Russia has inflicted pressure on Ukraine since an uprising nearly a decade ago toppled a government that resisted calls to move closer to the West.

Moscow seized the Crimean peninsula in 2014 when a pro-Russian insurgency broke out in eastern Ukraine that has since claimed more than 13,000 lives.

US officials say that Russia appears to be following a playbook from 2014 when it also sought to whip up sentiment with allegations of abuse by Ukraine.

The US ambassador to NATO, Julianne Smith told reporters in Brussels that there remained "an array of scenarios" possible on the ground, including a "full-scale conventional military attack".

Footage published by the Russian defence ministry today showed Russian tanks and infantry carrying out firing drills near the city of Rostov-on-Don in southern Russia near Ukraine.

Moscow said it 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 yesterday 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.

Mr Ryabkov also said he did not rule out the possibility that Moscow could deploy forces to allies Venezuela or Cuba if diplomacy failed.

Ukraine, however, renewed hope for diplomacy and said it has proposed a three-way video conference involving its president, Volodymyr Zelensky, and Mr Putin and Mr Biden.