"There is evidence that Russia continues to allow the free flow of weapons, funds, and fighters across its borders and President (Vladimir) Putin's next steps are still not clear," Treasury Secretary Jacob Lew said, in a speech in Washington.
Lew said the United States had worked with the International Monetary Fund, the World Bank and its G7 partners to coordinate a response to the crisis and provide Ukraine with financial and technical assistance.
Lew said the Treasury's coordinated and precise sanctions had put "enormous pressure" on Russia, with limited collateral damage to the US, European and global economies.
President Barack Obama, he said, "has given us the authority to take even more powerful action if Russia continues to support armed separatists in eastern Ukraine."
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Russia has insisted it is not destabilising Ukraine and has demanded that Kiev halt its military operations in the eastern part of the country. Fighting in the region has left nearly 200 dead -- soldiers, rebels and civilians -- since it broke out on April 13.
"Currently, Iran is at the negotiating table for the first time in a decade and progress on Iran's nuclear program has been halted while key elements of this program have been rolled back," he said.
"All options remain on the table to keep Iran from acquiring a nuclear weapon but we have a chance to resolve our differences peacefully and without the use of force."
Iran denies allegations it is developing a nuclear weapon, insisting the aims of its nuclear program are entirely peaceful.