LONDON — The top diplomats for Russia and the U.S. voiced pessimism today about negotiating an immediate end to the crisis in Ukraine or preventing its Crimea region from voting this weekend to break off from the rest of the country.

Headed into a morning meeting in London, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov each said they were glad to talk before the Sunday referendum in Crimea. The vote on the strategic Black Sea peninsula of 2 million people is widely expected to back secession and, potentially, annexation with Russia.

The U.S. and EU say the Crimea vote violates Ukraine’s constitution and international law. Russia has said it will respect the results of the referendum.

The upstart government in Kiev believes the vote is illegal, but Moscow says it does not recognize the new government’s leader as legitimate.

If Crimea secedes, the U.S. and European Union plan to slap sanctions as early as Monday on Russian officials and businesses accused of escalating the crisis and undermining Ukraine’s new government.

“This is a difficult situation we are in,” Lavrov said before his meeting with Kerry at the U.S. ambassador’s residence. “Many events have happened and a lot of time has been lost, so now we have to see what can be done.”

Kerry said he hoped to unearth “possibilities that we may be able to find about how to move forward, together, to resolve some of the differences between us.”