By Richard Balmforth and Mike Collett-White KIEV/SIMFEROPOL, Ukraine (Reuters) - Pro-Russian leaders in Crimea made final preparations on Saturday for a referendum widely expected to transfer control of the Black Sea region from Ukraine to Moscow, despite an outcry and threat of sanctions from the West. Russia vetoed a U.N. Security Council resolution that declared the referendum invalid, as Ukraine's defense ministry scrambled aircraft and paratroopers to confront what it said was a Russian encroachment just beyond Crimea's formal regional boundary. Ukraine's new rulers accused "Kremlin agents" of fomenting violence in the Russian-speaking east of the country. They urged people not to respond to provocations that Kiev fears Moscow may use to justify further incursions after its takeover of Crimea.