Helpless US can only fume

The White House warned President Vladimir Putin on Sunday that Moscow would face sanctions in coming days and international isolation that would hurt Russia's economy.

But Washington and the EU, though furious about the secession referendum in Ukraine's Crimea region, are unwilling to try to stop the Russian annexation by other than diplomatic means, and limited financial and travel sanctions.

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

With yesterday'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 US resolve to prevent Putin from "hijacking" Crimea.

Obama appears intent on showing that he was not bluffing when he threatened 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 US would not accept the results of Crimea's referendum on seceding from Ukraine and he once again urged a political resolution on Moscow, a State Department official said. But Moscow has treated all such calls with contempt.

Pfeiffer said the administration was working with its 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 in the coming days," Pfeiffer told NBC as the administration prepares to identify Russians whom the US will punish with visa bans and asset freezes Obama authorised last week.

A US sanctions announcement could come as early as today, a knowledgeable source said.

Foreign ministers from the EU, which has major trade ties with Russia, will decide on their actions in Brussels today.

Washington and its allies have ruled out military involvement.

US and EU officials worked over the weekend preparing lists of those to be targeted by sanctions.

Sanctions are not expected to be imposed on Putin himself at this point, and a US congressional source said the first round might also spare the Russian oligarchs close to him.

Though news reports have cited some of Putin's senior aides as targets, US and European officials could decide to start with lower-level Russian officials 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 US and Europe, and a freeze on bank accounts and other assets.

The Obama administration knows that Russia will retaliate. And efforts to punish Moscow are complicated by the need for its cooperation on reining in Iran's nuclear ambitions and the removal of Syrian chemical weapons. Permission to cross Russian territory is necessary for the US withdrawal from Afghanistan.

In Sunday's interview, Pfeiffer sidestepped the question of whether Washington would give military aid to Ukraine's interim government.

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

Republican senator John McCain, just back from Ukraine, called for US military assistance to Ukraine, resumption of the development of a US missile defence system for eastern Europe and accelerated steps towards Nato membership for Georgia and Moldova.

"The US has to make a fundamental reassessment of its relationship with Putin," McCain said.