(Adds White House statement, comments by State Department official, sanctions details)
By Matt Spetalnick and Arshad Mohammed
WASHINGTON, March 16 (Reuters) - The United States warned Russia on Sunday that Western sanctions were imminent and Moscow would pay an increasing price for its military intervention in Ukraine as the White House rejected a referendum in the Crimea region that it was powerless to stop.
With Washington and its European allies expected to unveil coordinated punitive measures against Moscow as early as Monday, U.S. Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov that Russia must pull its forces in Crimea back to their bases.
Confirming what people on both sides of the crisis had seen as a foregone conclusion in the hastily called referendum in Crimea - a region with a Russian-speaking majority - Russian state media said Crimeans voted overwhelmingly to break with Ukraine and join the Russian Federation on Sunday.
"This referendum is contrary to Ukraine's constitution," White House Press Secretary Jay Carney said. "The international community will not recognize the results of a poll administered under threats of violence and intimidation from a Russian military intervention that violates international law."
Amid accusations from Republican critics that President Barack Obama had not shown enough resolve against Russian President Vladimir Putin, the U.S. administration appeared intent on showing it was not bluffing in its threat of consequences for the seizure of Crimea.
U.S. options are limited to prevent Putin from formally annexing the strategic region, but some officials in Washington are hopeful that instead of pressing ahead with such a provocative action Putin may hold off for now, seeking to avoid further escalation of the situation.
White House senior adviser Dan 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 pressure to call it off.
"You can expect sanctions designations in the coming days," Pfeiffer told NBC's Meet the Press, as the administration prepared 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 is expected on Monday, and foreign ministers from the European Union, which has major trade ties with Russia, will decide on possible similar action in Brussels on Monday, Western sources said.
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.
Also not expected to be named in the latest round of sanctions are the heads of Russian oil company Rosneft or gas company Gazprom, with such figures possibly held in reserve if further punitive measures are needed.
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, removal of Syrian chemical weapons and use of Russian territory for U.S. withdrawal from Afghanistan.
Obama's aides have discussed the potential for "blowback" in other areas of U.S.-Russia relations and believe that Moscow will not take dramatic steps on issues unrelated to Ukraine.
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.
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 administration to provide military assistance to Ukraine, resume development of a U.S. missile defense system for Eastern Europe and take 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.
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.
In his talk with Lavrov, Kerry said Russian forces must return to base and objected to its military activities in Ukraine's Kherson oblast area and continuing "provocations" in Eastern Ukraine, a senior State Department official said.
Lavrov, according to a Russian statement, "agreed to continue work to find a resolution on Ukraine through a speedy launch of constitutional reform."
The U.S. official made clear Washington was pleased by Moscow's emphasis on constitutional reforms, describing this as "positive." The official emphasized, however, that what troubled the United States most were recent Russian troop movements. (Additional reporting by Toni Clarke, Bill Trott, Lesley Wroughton, Roberta Rampton and Andy Sullivan in Washington, Joshua Schneyer in New York; editing by Rosalind Russell and Matthew Lewis)