The people in Ukraine's Crimea region are hours away from voting on whether to become part of Russia. 

The U.S. and its European allies say the referendum set for Sunday is illegal and unconstitutional because Russian troops are now in control of Crimea. 

U.S. officials expect Crimean voters will choose to become part of Russia. 

They also say if Russia doesn't make an immediate effort to negotiate with Ukraine on resolving the crisis, stiff sanctions against Russia could happen as soon as Monday.