Russia's Federal Security Service said today it had thwarted two armed Ukrainian attempts to get saboteurs into Crimea and dismantled a Ukrainian spy network inside the annexed peninsula.

The FSB accused Ukrainian special forces of planning to carry out terrorist attacks inside Crimea targeting critical infrastructure and said an FSB employee and a Russian soldier had been killed in clashes with Ukrainian forces.

"The aim of this subversive activity and terrorist acts was to destabilise the socio-political situation in the region ahead of preparations and the holding of elections," the FSB said in a statement.

Russia annexed Crimea from Ukraine in 2014 in an operation that triggered U.S. and European Union sanctions. Ukraine says it wants the strategically-important peninsula back; Moscow says the matter is settled and closed forever.

The FSB said it had tackled one group of saboteurs in an operation that spanned Saturday night and the early hours of Sunday morning and that an FSB employee had been killed while trying to detain them.

It said it had found 20 homemade explosive devices, ammunition, mines, grenades and specialised weapons it said were commonly used by Ukrainian special forces.

It said it had detained a Ukrainian spy network in the process, detaining citizens of both Ukraine and Russia.

In the early hours of Monday, it said Ukraine had attempted to infiltrate two groups of saboteurs into Crimea by force, but that the FSB and other agencies had repelled the attempts.

It said one Russian soldier had been killed in what it called a "massive fire fight" when Ukrainian forces with the support of armoured vehicles had tried and failed to break into Crimea.

Security had been beefed up in areas popular with tourists, at key infrastructure, and along the border between Crimea and rump Ukraine, the FSB said.