US President Joe Biden has said that any entry of Russian troops into Ukraine will be treated by the West as "an invasion," as he tried to clarify confusion over an earlier suggestion that a "minor" attack could invite a lesser response.

"If any, any assembled Russian units move across the Ukrainian border, that is an invasion," Mr Biden said, adding he had been "absolutely clear" with his Russian counterpart Vladimir Putin.

"That will be met with severe and coordinated economic response that I have discussed in detail with our allies, as well as laid out very clear with President Putin," President Biden told reporters in the White House.

"Russia will pay a heavy price," he said.

However, expanding on comments he made on Wednesday that a "minor incursion" by Russia would be treated differently, Mr Biden said that Moscow would not necessarily mount a standard invasion and that "we have to be ready".

"That's also not the only scenario that we need to be prepared (for). Russia has a long history of using measures other than overt military action to carry out aggression. Paramilitary tactics, so-called gray-zone attacks and actions by Russian soldiers not wearing Russian uniforms," Mr Biden said.

He said Moscow also frequently uses cyberattacks.

"We have to be ready to respond to these as well in a decisive and united way with the range of tools at our disposal," the president said.

Ukraine's President Volodymyr Zelensky tweeted today that he wanted "to remind the great powers that there are no minor incursions".

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President Biden sought to reassure Kiev directly, saying: "The Ukrainian foreign minister said today that he is confident of our support and resolve - and he has a right to be."

Russia denies it is planning an attack, but says it could take unspecified military action if a list of demands are not met, including a promise from NATO never to admit Kiev as a member.

Ukrainian soldiers patrol the trenches along the frontline near the town of Zolote-4

US Secretary of State Antony Blinken met ministers from Britain, France and Germany in Berlin today.

"No matter which path Russia chooses, it will find the United States, Germany, and our allies, united," he said at a press conference with German Foreign Minister Annalena Baerbock.

"We urgently demand that Russia takes steps towards de-escalation. Any further aggressive behaviour or aggression would result in serious consequences," Ms Baerbock told the news conference.

Other allies made similar remarks, some clearly with Mr Biden's comments in mind.

A new attack would be met "with massive economic and financial sanctions. The transatlantic community stands firm in this," said European Commission President Ursula von der Leyen, who heads the EU executive.

British Prime Minister Boris Johnson said: "Be in no doubt that if Russia were to make any kind of incursion into Ukraine, or on any scale, whatever, I think that that would be a disaster, not just for Ukraine, but for Russia."

Moscow, for its part, said US threats of sanctions were not calming the situation.

A satellite image made available by Maxar Technologies shows a view of battle groups and a vehicle park in Yelnya, Russia

'Slip of the tongue'

With Western countries having long emphasised their united position in public, some officials privately expressed frustration at Mr Biden's remarks, although they described them as a gaffe, unlikely to alter Moscow's calculations.

"It was not helpful, in fact it was a gift to Putin, but we should not read too much into it. Biden has not given Moscow the green light for an attack on Ukraine. It was a slip of his tongue, and the official Western position will prevail," said one Western security source.

Another said: "I think the Russians will know how to rank Biden's remarks, they will classify it as what it was – a gaffe."

Moscow presented the West with a list of security demands at talks last week, including a ban on Ukraine joining NATO, that produced no breakthrough.

Washington has rejected Moscow's demands as "non-starters" and NATO chief Jens Stoltenberg this week insisted that the alliance "will not compromise on core principles such as the right for each nation to choose its own path".

Upping the ante, Russia announced new naval drills that will see it deploy to the Atlantic, Pacific, Arctic and Mediterranean "more than 140 warships and support vessels, more than 60 aircraft, 1,000 pieces of military equipment and about 10,000 servicemen".

High price

The West has repeatedly warned Russia it would pay a "high price" of economic and political sanctions should it invade Ukraine.

Western countries have imposed repeated rounds of economic sanctions since Russian troops seized and annexed Ukraine's Crimea peninsula in 2014.

Ukraine has since been fighting Moscow-backed forces in two breakaway eastern regions.

More than 13,000 people have been killed, and the latest Russian troop build-up has also greatly rattled neighbours in the Baltics.

But such moves have had scant impact on Russian policy, with Moscow, Europe's main energy supplier, calculating that the West would stop short of steps serious enough to interfere with gas exports.

US and European officials say there are still strong financial measures that have not been tried.

Germany has signalled that it could halt Nord Stream 2, a new gas pipeline from Russia that skirts Ukraine, if Moscow invades.

"Gas is not flowing through Nord Stream 2 yet, which means the pipeline is leverage for Germany, the United States and our allies, not Russia," Mr Blinken said at today's news conference.

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