(Adds Kerry-Lavrov call, sanctions details)
By Matt Spetalnick
WASHINGTON, March 16 (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, President Barack Obama's Republican critics accused the administration of showing weakness in the Ukraine crisis and said now was the time for U.S. resolve to prevent Putin from formally annexing Crimea.
Obama appears intent on showing he was not bluffing in his threat that Russia would pay a price for its seizure of Crimea, but his options are limited.
Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov in a telephone call on Sunday that the United States would not accept the results of Crimea's referendum on seceding from Ukraine and it continued to urge a political resolution on Moscow, a State Department official said.
Pfeiffer 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 U.S. and European threats against Moscow.
"You can expect sanctions designations in the coming days," 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 authorized last week.
A U.S. sanctions announcement could come as early as Monday, a source close to the matter said. Foreign ministers from the European Union, which has major trade ties with Russia, will decide on possible similar action in Brussels on Monday.
While Washington and its allies essentially have ruled out military action, American and European Union officials worked over the weekend preparing coordinated lists of those to be targeted initially.
Sanctions are not expected to be imposed on Putin himself at this point, and a congressional source said the first round could also spare Russian oligarchs close to him.
Though news reports have cited some of Putin's senior aides as possible targets, U.S. and European officials could decide to start mostly with lower-level Russian officials seen as complicit in the takeover of Crimea.
"Otherwise you leave yourself with no room to escalate" the allied response, the congressional source said. The measures would include bans on travel to the United States and Europe and a freeze on bank accounts and other assets held in those places.
At the same time, the Obama administration is mindful that Russia could retaliate with steps of its own. Any efforts to punish Moscow are complicated by the need for cooperation on Iran nuclear diplomacy, the removal of Syrian chemical weapons and use of Russian territory for the U.S. withdrawal from Afghanistan.
In Sunday's television interview, 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," Pfeiffer said.
He called on Congress to pass an economic aid bill for Ukraine that has stalled due to political wrangling.
Pfeiffer said 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 U.S. 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," 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. (Additional reporting by Toni Clarke, Bill Trott, Lesley Wroughton and Andy Sullivan in Washington, Joshua Schneyer in New York; editing by Rosalind Russell and Matthew Lewis)