New satellite images show that Russia has continued to build up its forces in annexed Crimea and near Ukraine in recent weeks.
The images by private company Maxar Technologies of a base in Crimea, which Russia annexed from Ukraine in 2014, show the Pogonovo training area in Voronezh packed with hundreds of armoured vehicles and tanks
Russia has concentrated an estimated 125,000 troops with tanks and other heavy weapons near Ukraine in what the West fears could be a prelude to an invasion.
The Kremlin says it reserves the right to move its own forces on Russian territory as it sees fit.
Meanwhile, the president of Ukraine has rebuked Joe Biden for a gaffe which was described as an "invitation for Putin to invade".
Volodymyr Zelensky warned against distinguishing between a small or large Russian attack on his country as he criticised the US leader for saying a "minor incursion" may not trigger a Nato response.
"We want to remind the great powers that there are no minor incursions and small nations," Zelesnky said on Thursday.
"Just as there are no minor casualties and little grief from the loss of loved ones. I say this as the President of a great power."
At a White House press conference on Wednesday, Biden said he now believed that Vladimir Putin will "move in" on Ukraine, and reiterated warnings that such an attack would have "disastrous" consequences for Russia.
But he added: "It's one thing if it's a minor incursion, and we [in Nato] end up fighting about what we should do, not do."
Asked if he was giving Putin "permission" for a "minor incursion," Biden said: "It did sound like that didn't it."
The remarks drew a furious response from Kyiv, with one official telling CNN it gave Putin a "green light" to invade.
Dmytro Kuleba, Ukraine's foreign minister, played down divisions, saying there was no question about Biden's commitment to Ukraine.
But he warned against repeating the "mistakes of 2014," when Russia's use of unmarked troops to annex Crimea and covert support for a war in the east of Ukraine succeeded in spreading confusion in the West about what it was doing.
"If we are speaking about a military invasion, there can be no distinction between small or large. Russia is already on the territory of Ukraine and is already killing Ukrainian soldiers," he told reporters in Kyiv.
"We won't accept attempts to explain to Ukraine that there is no need to do anything to restrain Russia, because it is not a significant enough invasion, or there is not enough proof these are Russian soldiers," he said.
The White House said in a clarifying statement that "if any Russian military forces move across the Ukrainian border, that's a renewed invasion, and it will be met with a swift, severe, and united response from the United States and our Allies."
The Kremlin said Biden's remarks that an attack would be a "disaster" for Russia could destabilise the situation.
Boris Johnson, the UK prime minister, reiterated Mr Biden's warning yesterday, saying: "any kind of incursion into Ukraine, on any scale whatever ... would be a disaster, not just for Ukraine, but for Russia."
Russia has deployed upwards of 125,000 troops near its border with Ukraine in what the Western government say is preparation for an invasion.
Putin has warned he may take military action unless Nato delivers on a number of security demands and the West forces Ukraine to make concessions on a 2015 peace deal.
Anthony Blinken, the US secretary of State, is due to hold talks with Sergei Lavrov, Russia's foreign minister, on Friday in what could be the last chance for a peaceful solution for the crisis.
He was in Berlin on Thursday to confer with his German, British, and French counterparts in Berlin in a last minute diplomatic scramble to maintain Nato unity and avert a conflict.
The diplomatic effort has been accompanied with an increase in military assistance to Ukraine as fears mount an attack is imminent.
A State Department official with Blinken's delegation in Berlin said the US was "expediting authorised transfers of US-origin equipment from other allies" after Lithuania said it wanted to send some of its American-made weapons to Ukraine.
"European allies have what they need to move forward on additional security assistance (to) Ukraine in the coming days and weeks," the official said.
Britain began airlifting short-range anti-tank missiles to Ukraine to strengthen its defences against a possible ground attack.
The meeting in Berlin came as Emmanuel Macron, the president of France, was accused of sowing transatlantic divisions by suggesting the European Union countries should "conduct their own dialogue" with Russia.
Macron, who wants the EU to beef up its defences to become less reliant on Nato and the US, said in a speech to the European Parliament that the bloc must make its own offer on Russia's security demands.
"We should build as Europeans working with other Europeans and with Nato and then propose it for negotiation with Russia," he said.
The Telegraph understands Paris had failed to inform European capitals that Macron would make the remarks, which broke away from a previous agreement by EU foreign affairs ministers to present a "strong, clear and united transatlantic front".
"It is, of course, not the first time that we are surprised by uncoordinated ideas from the big two (France and Germany)," a senior EU diplomat told the Telegraph.
A second European official said the comment "sends a signal of division, which is sad because there really isn't any in reality. Europe and the US are incredibly synced up on this".
"They will love this in Moscow and that is why it is dangerous," they added.
- additional reporting Telegraph