But US Secretary of State John Kerry also insisted Washington was not looking for a confrontation with Moscow amid the political upheaval in the former Soviet satellite.
"A lot of people are looking at this and trying to define this in the context of the Cold War, the old fights. But I want to underscore to everybody that this is not 'Rocky 4'. It is not a zero-sum game," Kerry told a small group of reporters, including AFP.
"We have only one interest and that is for the people of Ukraine to be able to make their choice, and to choose the path that they want to go down and for the rest of us to respect that path."
He insisted the top priority for the interim Ukraine leadership that has taken over after the ouster of president Viktor Yanukovych was forming a new inclusive government, as he revealed that the United States was "formulating initially a $1 billion loan guarantee with some other pieces" in aid.
The European Union was also looking at offering $1.5 billion in loan guarantees to the former Soviet satellite, he said.
The top US diplomat warned Russia that "any kind of military intervention that would violate the sovereignty, the territorial integrity, of Ukraine would be a grave mistake."
Although he was reluctant to say what the international response would be to any such military steps, he hinted there could be moves to take any such intervention to the United Nations.
Russian Defense Minister Sergei Shoigu said earlier Wednesday that Moscow was taking measures to ensure the security of its Black Sea naval fleet based on Ukraine's southern Crimean peninsula.
Sevastopol, which has hosted the Black Sea fleet since tsarist times, has witnessed several pro-Russian demonstrations in recent days with hundreds rallying outside city hall on Tuesday.
"For a country that has spoken out so frequently in the last year ... and so strongly against foreign intervention in Libya and Syria and elsewhere, it would be important for them to heed those warnings as they think about options in the sovereign nation of Ukraine," Kerry told the reporters roundtable in his private conference room.
- 'Ukraine must form new government' -
He insisted however: "Personally I don't think it's enough for us to be heralding the advent of democracy and applaud the courage of conviction of the people who brought about this transition and then just not doing anything. I think that's unconscionable."
He said it was urgent to move forward, adding: "I think everybody's on the same page, I think what we're just looking at is what's the appropriate level and how do we go at this."
Apart from guaranteeing up to $1 billion in loans from global financial institutions such as the World Bank and the International Monetary Fund, Washington was also looking at providing some direct assistance to Kiev.
But this was still to be decided upon and "the president has to decide what the level is that he thinks is appropriate to ask for."
"Priority number one is forming the new government, forming it as fast as possible in as broad based a coalition as they can build. Priority number two is reforms, instituting reforms, so that you can lay the groundwork for the IMF. And priority three is the IMF," Kerry added.
A White House spokesman, Josh Earnest, meanwhile said "outside actors" should "end provocative rhetoric and actions" -- in an apparent reference to Russia.
He also told reporters on Air Force One that all governments should keep commitments to transparency about military activities under the Vienna Document of 2011 covering Europe and Central Asia and other Organization of Security and Cooperation in Europe obligations designed to ensure security and peace.
"It is important for all of the parties in the region, both the Ukrainian government as well as the Russians, (to) understand that it is important for them to live up to their obligations."