The headquarters of Bank Rossiya used by close associates of President Vladimir Putin in Saint-Petersburg, on March 21, 2014

If Moscow does not stop interfering in Ukraine and supporting pro-Russia separatists, the US will implement more sanctions, he said.

"Our approach is a calibrated effort to impose immediate costs on Russia and to create conditions that will make Russia increasingly vulnerable to sanctions as the situation in Ukraine escalates," Glaser said.

"Russia is already feeling the impact of our measures."

The impact so far, he said, includes:

-- A 13 percent fall in the Russian stock market

-- The Central Bank of Russia being forced to spend almost $50 billion to support the ruble

-- A jump in Russia's borrowing costs, forcing Moscow to cancel a debt auction on April 23

-- Capital flight from Russia in the range of $100-130 billion this year

-- Rossiya Bank, the Putin-linked bank listed for sanctions, lost $1 billion in deposits since March and was forced to sell $500 million in bonds to maintain liquidity.

Glaser acknowledged that Washington’s European allies were more cautious about sanctions given their greater economic and financial exposure to Moscow.

But he said: "As the Kremlin's decisions concerning the situation in Ukraine leave us with little choice but to continue to ratchet up the pressure, we will use the full range of sanctions authorities at our disposal, which will expose the weakness and vulnerability of the Russian economy."