The US sees no indications that Russian forces along the border with Ukraine are carrying out the kind of military exercises that Moscow has cited as the reason for their deployment.
Ukraine's government has put its heavily outnumbered forces on alert following Moscow's seizure of Crimea.
US and European security agencies estimate Russia has deployed military and militia units totalling more than 30,000 people along its border with eastern Ukraine.
The Pentagon has cited assurances from Russian Defence Minister Sergei Shoigu that its troops along the border were sent for exercises and that they would not cross into Ukraine.
However, US officials have acknowledged concerns about continued Russian reinforcements to the area.
"We've seen no specific indications that these – that exercises - are taking place," Pentagon spokesman Rear Admiral John Kirby told a news briefing.
But when pressed, Admiral Kirby added: "Just because we haven't seen an indication of exercises now doesn't mean that one won't occur."
"(The Russians) made it clear that their intent was to do exercises and not to cross the border.
"Our expectation is they're going to live up to that word," he said.
Britain's Defence Secretary Philip Hammond, speaking at the Pentagon yesterday, appeared to give little weight to any assurances from Mr Shoigu, saying Russian President Vladimir Putin seemed to be calling all the shots.
"Other Russian players, including Minister Shoigu, may express views, but it's a moot point. And we cannot know, we do not know, to what extent all of those people are really inside the inner circle in which President Putin is planning this exercise," Mr Hammond said.
US and European sources say that US and European government experts believe that there has been, and continues to be, a steady and noticeable build up in the total number of Russian forces along the Ukrainian border, though some military units have rotated in or out of the area.
The sources said that the Russian force deployed along the Ukraine border includes regular military including infantry as well as armoured units and some air support.
Also deployed are militia or special forces units comprised of Russian fighters, wearing uniforms that lack insignia or other identifying markings, similar to the first Russian forces to move into Crimea during Russia's recent military takeover there.
IMF agrees bailout package for Ukraine
The International Monetary Fund said it had agreed a $14-18 billion bailout for Ukraine, a deal that will unlock further credits to reach a total of $27 billion over the next two years.
The agreement is intended to help Ukraine meet debt payments looming this year after months of anti-government protests.
"The mission has reached a staff-level agreement with the authorities of Ukraine on an economic reform programme that can be supported by a two-year Stand-By Arrangement (SBA) with the IMF," the IMF said in a statement.
"The financial support from the broader international community that the programme will unlock amounts to $27 billion over the next two years.
"Of this, assistance from the IMF will range between $14-18 billion, with the precise amount to be determined once all bilateral and multilateral support is accounted for."
The agreement is subject to approval by IMF Management and the Executive Board, which will consider it in April.
"Following the intense economic and political turbulence of recent months, Ukraine has achieved some stability, but faces difficult challenges," the IMF said.
Announcing the agreement in the Ukrainian capital, IMF mission chief Nikolay Gueorguiev declined to say how big the initial tranche of aid would be.
Ukraine has said it desperately needs cash to cover expenses and avert a possible debt default.
The country's finance minister has predicted the economy will contract 3% this year, weakened by years of mismanagement and political turmoil.
The bailout from the IMF will help prop up Ukraine's economy and clear the way for several billion dollars in aid from the United States, European Union, Japan and other nations.
Ukraine's new leaders yesterday announced a radical 50% increase in the price of domestic gas from 1 May.
Tymonshenko to run for Ukrainian presidency
Former Ukrainian prime minister Yulia Tymoshenko, released from jail last month after former president Viktor Yanukovych fled from power, has announced she would run again for president in an election on 25 May.
Ms Tymoshenko, 53, served twice as prime minister and ran for president in 2010, only to be narrowly beaten in a run-off vote by Mr Yanukovych.
Mr Yanukovych subsequently launched a campaign against Ms Tymoshenko and her allies and she was jailed in 2011 for abuse of office linked to a gas deal she brokered with Russia in 2009.
She served two years of a seven-year term, mainly under prison guard in a hospital in Kharkiv, before being released when Mr Yanukovych fled on 20 February and was subsequently ousted by parliament.