Russia has accused Ukraine of trying to carry out armed incursions into Crimea – the territory annexed by Russia in 2014 after an unrecognised referendum.
The FSB intelligence agency said two attempted incursions had taken place over the weekend and a Russian soldier and an FSB employee had been killed.
President Vladimir Putin vowed “further security measures” in response to Ukraine’s “stupid and criminal” acts.
Ukraine’s president described the accusations as “preposterous”.
“Russian accusations against Ukraine of terrorism in occupied Crimea sound as preposterous and cynical as the statements of the Russian leadership about the absence of Russian troops in the Donbass [region of rebel-held eastern Ukraine],” Petro Poroshenko said.
“These fantasies are only a pretext for making more military threats against Ukraine,” he added.
Russian intelligence also said it had smashed a Ukraine military intelligence network in Crimea and detained a number of people. They included a Ukrainian national named as Yevhen Panov, who is described by Russian sources as a Ukrainian military intelligence officer.