Ukrainian leaders dispatched on an emergency diplomatic mission to Crimea on Wednesday were told by the peninsula's leader that they would be turned back, Russias's Interfax news agency reported.
Ukrainian Prime Minister Arseny Yatseniuk ordered his deputy, Vitaly Yarema, and acting Defense Minister Ihor Tenyukh to Crimea to "resolve the situation" after the region was effectively annexed by Russia on Tuesday.
But Crimea's Prime Minister Sergei Aksyonov told Interfax that the ministers would be barred from entering.
"They are unwelcome in Crimea," he said. "Nobody will let them enter Crimea, and they'll be sent back."
Aksyonov was appointed as prime minister last month in the days after Kiev's popular uprising and regime change, which he rejected. He was voted in by fellow lawmakers while the Crimean parliament was under occupation by unidentified gunmen.
Russia's constitutional court said Wednesday that the draft treaty annexing Crimea was legal, meaning the document needs only to be ratified by Moscow's parliament to become official. However this is a formality, and Russia said the annexation became effective as soon as it was signed by President Vladimir Putin.
Just hours later the Ukrainian military reported that one its personnel had been shot dead at a military base in Crimea, although this was contradicted by Russian reports that said it was a pro-Russian militiaman who died.