The Obama administration has signalled that it no longer recognises ousted Viktor Yanukovych as Ukraine's president.
The shift of support for opposition leaders in the capital Kiev came even as US officials sought to assure Russia that it does not have to be shut out of a future relationship with a new Ukrainian government.
Mr Yanukovych was widely seen as a puppet of Moscow against Ukraine protesters who demanded stronger ties with the European Union to boost the faltering economy of the one-time Soviet republic.
His whereabouts were unknown after he fled Kiev in the wake of deadly protests seeking his removal.
US officials said the International Monetary Fund was considering an aid package as high as 15 billion dollars (£9bn) to help stabilise a new, transitional government in Kiev.
White House spokesman Jay Carney said America would provide additional aid to complement the IMF, aimed at fostering Ukrainian economic stability, but it was not immediately clear how much money it would provide. Officials later said any US assistance would seek to help Ukraine implement political reforms, in part though investing more in health and education.
"Yanukovych left Kiev. He took his furniture, packed his bags, and we don't have more information on his whereabouts," State Department spokeswoman Jen Psaki said. "So there are officials who have stepped in and are acting in response to that leadership gap at the moment."
Mr Carney said that although Mr Yanukovych "was a democratically elected leader, his actions have undermined his legitimacy, and he is not actively leading the country at present".
Senior US officials, including deputy secretary of state William Burns and treasury secretary Jack Lew, will meet political, business and civic leaders during a series of meetings in Kiev over the next two days. European Union chiefs are already there.
Mr Psaki said Congress must approve any US aid package, and several politicians called yesterday for a quick show of support for Ukraine's new leaders.
"Now more than ever, the Ukrainian people need the continued support of their friends," Republican senator John McCain said. "The path of reform will be difficult, but if the new Ukrainian government is prepared to make these tough - and, at times, unpopular - decisions, it will need significant assistance from the IMF and the European Union. The United States must be ready to provide additional assistance as well."
The protests in Kiev were sparked by Mr Yanukovych's shelving of an agreement with the European Union in November and turning instead for a £9 billion bail-out loan from Russia. Within weeks the protests expanded amid outrage over corruption and human rights abuses, leading to calls for Mr Yanukovych's resignation.
Anger boiled over last week after 82 people, primarily demonstrators, were killed in clashes with security forces in the bloodiest violence in Ukraine's post-Soviet history.
Aides said US vice president Joe Biden spoke to Georgian prime minister Irakli Garibashvili in his first of several discussions about regional security, including with leaders of other former Soviet republics which are also struggling, either politically or economically, against generations of inherent Russian influence. But Mr Psaki said the emergence of a new government in Kiev was not "a zero-sum game for Russia or any other country".
"It's in all of our interests to support a prosperous future for the country," he said, underscoring the administration's intent to acknowledge Ukraine's desire to seek European aid and partnerships while still, perhaps, maintaining a productive relationship with Moscow.
But scepticism remains, including on Capitol Hill. Democratic congressman Adam Schiff urged Russia to refrain from interfering in Ukraine's transition, noting that the new government in Kiev would "face a months-long process" of reorganising and regaining the public's trust.
"Any meddling or economic extortion will not put Russia's chosen leader back in power or end the protests," Mr Schiff said. "But it will impair Ukraine's ability to heal its wounds, and Russia's efforts to improve its standing on the world stage."
Meanwhile, the Speaker of Ukraine's parliament said a new government should be in place by Thursday, a delay reflecting intense ongoing consultations.
Oleksandr Turchinov has previously said the new government could be formed today.
Mr Turchinov was named Ukraine's interim leader after Mr Yanukovych fled the capital after signing a peace deal with opposition leaders to end violent clashes between police and protesters and Kiev.
Law enforcement agencies have issued an arrest warrant for Mr Yanukovych over the killing of 82 people, mainly protesters. His whereabouts are unknown. He was last reportedly seen in the Crimea, a pro-Russia area.
The parliament sacked some of Mr Yanukovych's lieutenants and named their replacement, but it has yet to appoint the new premier and fill all posts.