More than 40 per cent of the Russian forces on the Ukraine border are now in position for attack, a US defence official said.
Satellite pictures show Russian troop tents and a field hospital have remained in place in Novoozernoye, Crimea.
More than 40 per cent of the Russian forces on the Ukraine border are now in position for attack and Moscow has begun a campaign of destabilization, a US defence official said.
The United States, which estimates that Russia has placed more than 150,000 troops near Ukraine's borders, has observed significant movements, the official said, insisting on anonymity.
A close-up satellite picture shows Russian helicopters deployed at Lake Donuzlav, Crimea, near Novoozerne.
Moscow denies it has plans to attack its western neighbour, but is demanding a guarantee that Ukraine will never join NATO and that the Western alliance remove forces from Eastern Europe, demands the West has refused.
In 2014, Russia invaded and occupied the Crimea region of Ukraine, making use of sympathetic separatists.
Satellite images shows an overview of helicopters deployed at Lake Donuzlav, Crimea, near Novoozerne.
Possible Russian recognition of separatist "republics" independent from Kyiv is threatening to derail an already fragile peace process, as fears grow of large-scale conflict in eastern Ukraine.
Tensions between the West and Russia over Ukraine have soared in recent months, after Moscow massed tens of thousands of troops near its neighbour's border.
Western leaders say Russia could launch an attack on Ukraine at any moment, though Moscow has denied such plans.
Russia's leader will oversee major military drills along Ukraine's borders on Saturday, further escalating tensions after Washington said Moscow would invade within days, and Ukraine's president headed to Europe to drum up support.
Artillery shelling in the east of Ukraine and orders from Russian-backed separatists for civilians to evacuate the region Friday inflamed an already febrile situation as Washington insisted Moscow was encircling its pro-Western neighbour.
The Kremlin continues to say it has no plans to attack.
Kyiv has been battling pro-Russia separatists in its eastern regions since 2014 in a conflict that has claimed around 14,000 lives.
Fighting has largely diminished since the 2014 and 2015 Minsk accords, under which Russia and Ukraine agreed to a ceasefire and a roadmap to a political settlement.
But that process has hit a wall, with each side accusing the other of not fulfilling its end of the deal.
The Minsk agreements provided for Donetsk and Lugansk to remain part of Ukraine, but for local elections to be held in the separatist regions under Ukrainian law, and for interim self-government in certain areas of them.
Foreign armed formations were to withdraw from those areas.
But Ukraine has not given the regions special status or held the polls, arguing that Russia must first end what it calls its covert military presence in the region.
After a brief ground assault into southern neighbour Georgia in 2008, Russia recognised the independence of the country's South Ossetia and Abkhazia regions, and established permanent Russian military bases there.
Putin has appeared to reject parliament's demand on eastern Ukraine, saying he wanted to implement the Minsk accords "to the end".
But he has also stressed that most Russians sympathised with Russian-speaking Donbas residents, whom he claims are the victims of a Kyiv-orchestrated "genocide".
Ukrainian negotiators however say Russia is itself hampering the peace process by insisting on a Moscow-brokered dialogue between Kyiv and the separatists.
Ukraine has refused to enter into such talks, arguing that Russia is an instigator of the conflict, not an impartial mediator.
"Sooner or later, Ukraine will introduce the draft laws" necessary to the peace process, said Sergiy Garmash, one of Kyiv's negotiators.
"But in view of the obstruction created by Russia's demands, it will take years to examine them."
Lukyanov, the Russian analyst, said Russia was turning up the pressure for Ukraine to carry out its promises.
The United States has dismissed reports that Russia was withdrawing troops from Ukraine's border, instead accusing Moscow of sending more soldiers as fears of an invasion grow.
Russia has increased its presence on the border with Ukraine by "as many as 7,000 troops," some of whom arrived Wednesday, said a senior White House official, slamming Moscow's announcement of a withdrawal as "false."
"We continue to receive indications they could launch a false pretext at any moment to justify an invasion."