US Secretary of State John Kerry has headed for last-ditch talks with Russia seeking to defuse tensions over Ukraine and warning Moscow of a serious backlash over Crimea's breakaway vote.
Just hours before he left on Thursday for London to meet again with Russian counterpart Sergei Lavrov ahead of Sunday's vote, the top US diplomat cautioned Moscow to move to help end the crisis.
"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us," Kerry warned.
The self-declared leaders of Crimea are poised to hold a referendum with the voters in the majority ethnic Russian southern peninsula expected to choose to split from Ukraine and join Russia.
Ukraine and Washington have rejected the vote as illegitimate. And both the US and EU are readying tough sanctions against those blamed for stirring the tensions in Crimea.
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Asked if he would argue with the notion that Friday's talks were Moscow's last chance to avoid a new slew of sanctions, White House spokesman Jay Carney replied "no."
Work was "being done to identify targets of potential sanctions" as well as possible "further costs" for Russia, Carney added.
Kerry told politicians the US administration did not want to impose further sanctions on Russia.
"Our choice is not to be put in the position of having to do that. Our choice is to have a respect for the sovereignty and independence and territorial integrity of Ukraine," Kerry said.
He revealed he had spoken again by phone with Lavrov on Thursday ahead of their Friday meeting, with the two top diplomats having been in almost daily contact in the past two weeks during the most bitter East-West showdown since the end of the Cold War.
He also hinted at a possible compromise under which the Ukrainian parliament could allow Crimea to hold a referendum on self-determination - such as Scotland's looming vote in September to end its 300-year old union with England.
"It's been done through the appropriate mechanism," Kerry said, referring to the British parliament's green-light to allow the referendum to go ahead.
The signs however are not good for Friday's talks, as Kerry and Lavrov have clashed in recent weeks and failed to reach accord on a number of US proposals.
Moscow has also refused to recognise the interim leadership in Kiev or take part in a contact group aimed at bringing the two countries together for talks.
