WASHINGTON, March 13 -- U.S. Secretary of State John Kerry on Thursday threatened "a very serious series of steps" against Russia with the European Union in response to Crimea's vote on Sunday to break away from Ukraine.

The top American envoy told some senators that Russia has an estimated 20,000 troops in Crimea, home to the Russian Black Sea Fleet, as the Russian-majority autonomous region is well certain to vote to leave Ukraine and join Russia in the referendum days away.

"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 the options that are available to us," Kerry said at a Senate panel hearing on his agency's budget for fiscal year 2015.

"Now our choice is not to be put in the position of having to do that," he added. "Our choice is to have a respect for the sovereignty and independence and integrity of the country of Ukraine. Our hope is to have Russia join in respecting international law."

Russian parliament leaders have said that Russia would respect the choice made in Crimea's plebiscite, while Washington and its European allies have rejected the upcoming vote as running contrary to the international law and Ukraine's constitution.

"I don't think there's much doubt, given the circumstances, what the vote is going to be," Kerry said. "Nobody doubts that. So this is not a question mark."

"The question mark is, is Russia prepared to find a way to negotiate with Ukraine, with the contact group, with the other countries involved in order to be able to resolve this in a way that respects their legitimate interests, and they have legitimate interests, but respects them in a way that doesn't violate international law and isn't at the butt of a rifle and of massive military imprint?" he added.

Russia does not recognize Ukraine's new government put in place after pro-Russian President Viktor Yanukovych was ousted on Feb. 22, and called the events in the neighboring country a coup.

Kerry is scheduled to leave for London later Thursday for talks with his Russian counterpart Sergei Lavrov, as U.S. President Barack Obama said on Wednesday he hoped diplomatic efforts over the next several days could cause a "rethinking" of plans for the referendum.

Washington has canceled trade talks and military exchanges with Russia, and Obama has authorized visa bans and assets freeze on some Russians and Ukrainians blamed for Moscow's military moves in Crimea.