Ukraine has denied the claims, describing them as 'fake information'
Russia has accused the intelligence agency of Ukraine's defence ministry of planning a number of incursions and 'terrorist attacks' in Crimea.
Russia’s Federal Security Service (FSB) says it foiled an attack by a group of 'sabouteurs' over the course of Saturday night and Sunday morning.
An FSB officer is said to have been killed during the operation, and Russian and Ukrainian citizens were detained.
Security forces say they found 20 improvised explosive devices, as well as mines, weapons, and ammunition.
Two more attempted attacks were reportedly thwarted on Monday.
According to Russian news agency RT, the FSB says Ukrainian armored vehicles provided covering fire for the 'terrorist groups'.
Officials say a Russian serviceman was also killed during that operation.
In a statement translated by Reuters, the FSB suggested: "The aim of this subversive activity and terrorist acts was to destabilize the socio-political situation in the region ahead of preparations and the holding of elections."
'Critical infrastructure' was apparently being targeted.
Additional security measures have been put in place by Russian authorities following the alleged incidents, and border controls increased.
A spokesperson for Ukraine's intelligence agency denied Russia's accusations, telling Reuters that "this is fake information".
The Crimea territory was annexed by Russia in 2014, which has led to increased tension between Ukraine and Russia. The annexation has also been widely condemned by international leaders.