Ukraine's premier Arseniy Yatsenyuk on Thursday dubbed as "illegitimate" a request by the local parliament in Crimea to become part of Russia.
"This is an illegitimate decision," Yatsenyuk told reporters after talks with European Union heads of state and government.
"We urge the Russian government not to support those who advocate separatism," he added.
Pro-Moscow lawmakers made the request to Russian President Vladimir Putin and said they would put the question to a referendum on March 16 as Russian forces maintained their grip on the strategic Black Sea peninsula.
The referendum, brought forward from a planned March 30 date, will also ask if residents want to a return to a previous 1992 constitution that gave the region sweeping autonomy.
"Crimea is and will be an integral part of Ukraine," Yatsenyuk said. "This so-called referendum has no grounds at all."