US vice-president Joe Biden promised Ukraine's pro-Western interim leaders "full" US support on Thursday as they seek to head off Russian influence.
Biden called interim prime minister Arseniy Yatsenyuk to "welcome the formation of a new government in Ukraine," a White House statement said.
The call came as Ukraine's new leaders issued a blunt warning to Russia against any aggression on the volatile Crimean peninsula.
Dozens of pro-Kremlin gunmen in combat fatigues have seized government buildings in the autonomous republic and anxious Western governments pleaded with Moscow not to escalate tensions.
The country's ousted pro-Moscow president Viktor Yanukovich, meanwhile, emerged defiant from five days in hiding, saying he still considered himself president.
Biden "emphasised that this is an important opportunity not only to bring peace, stability, and unity to Ukraine, but also to restore the faith of all of the Ukrainian people in their country's democratic institutions as they prepare for new elections in May," a White House statement said.
"The vice-president reassured the prime minister that the United States will offer its full support as Ukraine undertakes the reforms necessary to return to economic health, pursue reconciliation, uphold its international obligations and seek open and constructive relationships with all its neighbors."
Top US diplomat John Kerry said earlier that Russian foreign minister Sergei Lavrov told him that Moscow "will respect the territorial integrity of Ukraine."
Lavrov also insisted Moscow was not behind the storming of Crimean government buildings by dozens of armed pro-Kremlin gunmen, according to Kerry.
The White House also reinforced US warnings to Russia that it must avoid "miscalculations" in military drills along Ukraine's border.