The United States said Monday that Russia is continuing to support the pro-Russian insurgency in Ukraine, despite US sanctions aimed at punishing it for its alleged interference in its neighbor.

The United States said Monday that Russia is continuing to support the pro-Russian insurgency in Ukraine, despite US sanctions aimed at punishing it for its alleged interference in its neighbor.

"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.

"Our goal was to impose a cost on Russia for its occupation and attempted annexation of Crimea and to deter Russian military intervention in Ukraine," he said, according to prepared remarks for an event at the Center of Strategic and International Studies.

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."

Russia has insisted it is not destabilizing 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.

Lew also said that US-led international sanctions against Iran, to convince Tehran to abandon its program to develop a nuclear weapon, have produced positive results.

"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.