The United States and NATO have said Russia is still building up troops around Ukraine, despite Moscow's insistence it was pulling back.
Russia has said military drills in Moscow-annexed Crimea had ended and that soldiers were returning to their garrisons.
However, NATO said the troop buildup was continuing, questioning Moscow's stated willingness to negotiate a solution to the crisis.
US Secretary of State Antony Blinken backed NATO's assessment in a morning television interview.
"There's what Russia says and then there's what Russia does. And we haven't seen any pullback of its forces," Mr Blinken told MSNBC.
"We continue to see critical units moving toward the border, not away from the border. What we need to see is exactly the opposite. We need to see these forces moving away."
Tonight, a White House official said that Russia had added "as many as 7,000'" troops to Ukraine border.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
'So far, no de-escalation'
NATO Secretary-General Jens Stoltenberg said a pullout would be welcome, but that moving troops and tanks back and forth did not amount to proof it was really happening.
"We have not seen any withdrawal of Russian forces. And of course, that contradicts the message of diplomatic efforts," Mr Stoltenberg said before a meeting of the alliance in Brussels.
"What we see is that they have increased the number of troops and more troops are on their way. So, so far, no de-escalation."
It comes a day after Russia announced that a first troop pullback from Ukraine's borders had got under way.
"Units of the Southern Military District, having completed their participation in tactical exercises, are moving to their permanent deployment points," the defence ministry said in a statement.
State television showed images of military units crossing a bridge linking the Russia-controlled peninsula to the mainland.
The statement said tanks, infantry vehicles and artillery were leaving Crimea by rail.
US President Joe Biden made an impassioned appeal to Russian President Vladimir Putin to step back from war with Ukraine, speaking starkly of the "needless death and destruction" Moscow could cause and international outrage Mr Putin would face.
Mr Biden said the US estimates that 150,000 Russian troops now encircle Ukraine, a higher number than previous estimates of about 100,000.
He said reports that some forces had withdrawn were welcome, but they were unverified and an invasion remained very much a possibility.
Meanwhile, Ukrainian President Volodymyr Zelensky has urged citizens to celebrate a "Day of Unity".
Read More:
Ukraine hit by cyber attack, US questions Russian troop pullback
Ukraine looking for 'peaceful resolution' - Ambassador
Russia invading Ukraine 'insane' - Russian Ambassador
Yesterday, Russian President Vladimir Putin met Germany's Chancellor Olaf Scholz to explore a route to a negotiated solution and Moscow said it had begun to pull back some forces.
Western leaders have accused Moscow of positioning troops in advance of a possible invasion of Ukraine, warning that any attack would be met with severe economic sanctions.
After meeting Mr Scholz in Moscow, Mr Putin said Russia "of course" did not want war, and was willing to look for solutions with the West.
"We are ready to work further together. We are ready to go down the negotiations track," Mr Putin told a joint press conference with Mr Scholz.
In response, the German Chancellor said: "That we are now hearing that some troops are being withdrawn is in any case a good sign."
"For Europeans, it is clear that lasting security cannot be achieved against Russia but only with Russia."
The crisis - the worst between Russia and the West since the Cold War ended - reached a peak this week, with US officials warning of a full-scale invasion - perhaps today.
Mr Zelensky reacted with sarcasm to the warning, and declared today a "Day of Unity".
"Serious external and internal challenges have arisen, which require responsibility, confidence and concrete actions from me and each of us," he said.
"But our state today is stronger than ever," he vowed.
Ukraine said yesterday that the websites of the country's defence ministry and armed forces, as well as two banks, had been hit by a cyber attack that could have Russian origins.
"It cannot be excluded that the aggressor is resorting to dirty tricks," Ukraine's communications watchdog said, in reference to Russia.
In a separate move likely to anger Kyiv, Russian MPs voted to urge Mr Putin to recognise two breakaway regions in eastern Ukraine as "sovereign and independent states".
This would allow Russia to abandon the Minsk agreements peace plan for eastern Ukraine and potentially move in Russian troops - giving Mr Putin a strong hand to play in any future negotiations with Kyiv.
The European Union "strongly" condemned such a move, saying it would violate the Minsk agreements that Moscow had signed.
Russia has repeatedly blamed the Ukraine crisis on the West, saying the US and western Europe are ignoring Russia's legitimate security concerns.
The Kremlin insists NATO must give assurances that Ukraine will never be admitted as a member and the alliance roll back its presence in several eastern European and ex-Soviet countries.
Russia already controls the Crimean Peninsula that it seized from Ukraine in 2014 and supports separatist forces who have taken control of parts of eastern Ukraine, in a conflict that has claimed more than 14,000 lives.
Additional reporting: Paul Cunningham
