US Treasury Secretary Jacob Lew warned his Russian counterpart Thursday of more sanctions if Moscow "continues to escalate" the crisis in Ukraine.

Lew also expressed to Finance Minister Anton Siluanov concerns about reports that Moscow was discussing an oil-for-goods trade deal with Iran, the Treasury said in a statement.

On the sidelines of IMF and World Bank spring meetings in Washington, Lew told Siluanov "Russia's ongoing occupation and purported annexation of Crimea is illegal and illegitimate."

Lew told him that "in addition to the sanctions already imposed in response to Russia's occupation and purported annexation of Crimea, the United States is prepared to impose additional significant sanctions on Russia if it continues to escalate the situation in Ukraine."

Echoing calls from President Barack Obama's administration, Lew said the United States believed that Russia can resolve the dispute with Ukraine's new interim government diplomatically.

Obama has imposed sanctions on Russia, blacklisting officials, businesspeople and a bank close to President Vladimir Putin, to protest Moscow's seizure of Crimea from Ukraine.

The United States and its allies have also voiced concern as Russia steps up pressure on Ukraine's new pro-Western government.

Putin on Thursday threatened to cut off gas unless Ukraine pays $2.2 billion in debts.

Western nations have rallied to support Ukraine, with the International Monetary Fund crafting an aid package that would total $14 to $18 billion over two years.

IMF managing director Christine Lagarde said earlier Thursday that the Fund would submit the plan by early May and that she expected Ukraine to meet criteria on its economy's stability, despite concerns in some quarters about the new government's capacity.

The package's goal, Lagarde told reporters, will be for Ukraine to "restore its financial situation so that it can finance itself, refinance itself and operate without support from the IMF."

The IMF, which counts the United States as its biggest stakeholder but also has Russia as a member, has warned of economic risks for Moscow over the Ukraine crisis.

On Tuesday, the IMF slashed its growth forecast for Russia for the current year to 1.3 percent, and warned its economy may contract by as much as 1.8 percent if the crisis with Ukraine is prolonged.

Lew also voiced "serious concerns" over indications that Russia is negotiating an oil-for-goods deal with Iran.

The United States has eased limited sanctions on Iran after the Islamic republic agreed to freeze sensitive nuclear work as part of a tentative deal with six foreign powers.

But Obama has insisted only on a small let-up in sanctions before a final deal. US lawmakers and Israel have opposed any easing of pressure on Iran.