WASHINGTON (REUTERS) - The White House warned President Vladimir Putin on Sunday that Moscow would face sanctions in coming days and international isolation that will hurt Russia's economy, as Washington fumed over a referendum in Ukraine's Crimea region that it was powerless to stop.

"We are putting as much pressure on the Russians as we can to do the right thing," White House senior adviser Dan Pfeiffer said as voters in Crimea, under the control of Russian forces, decided whether to break away from Ukraine and join Russia.

With Sunday's referendum widely expected to favor union with Russia for a region that has a Russian-speaking majority, some of President Barack Obama's Republican critics accused the administration of showing weakness in the Ukraine crisis and said now was the time for United States resolve.

Mr Pfeiffer insisted the United States would not recognise the results of the referendum and said the administration was working with European partners to step up pressure on Russia in the worst East-West standoff since the Cold War. Crimea's pro-Russian regional government went ahead with the referendum despite US and European threats against Moscow.

"You can expect sanctions designations in the coming days," Mr Pfeiffer told NBC's Meet the Press, as the administration prepares to identify Russians whom the United States will seek to punish with visa bans and asset freezes the president authorised last week.

While the United States and its allies essentially have ruled out military action, Mr Pfeiffer sidestepped the question of whether Washington would provide military aid to Ukraine's interim government, which has accused Russia of violating its sovereignty over Crimea.

"We're looking at all ways of assistance," Mr Pfeiffer said.

He called on Congress to pass an economic aid bill for Ukraine that has stalled due to political wrangling.

Mr Pfeiffer said Mr Putin has a choice. "Is he going to continue to further isolate himself, further hurt his economy, further diminish Russian influence in the world, or is he going to do the right thing?" he said.

Republican Senator John McCain, just back from a visit to Ukraine, urged the Obama administration to do more. He called for military assistance to Ukraine, resumption of development of a US missile defense system for Eastern Europe and steps toward NATO membership for Georgia and Moldova.

"The United States of America has to first of all have a fundamental reassessment of our relationship with Vladimir Putin. No more 'reset' button," Mr McCain told CNN, referring to Obama's outreach to Russia early in his first term that has since been abandoned.

Tennessee Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee, told Fox News "there's no question our administration has created an air of permissiveness" by failing to take a tougher line with Russia.