The FSB said in a statement that its officer was killed late on Saturday outside the town of Armyansk within miles of the de-facto border between Crimea and Ukraine when FSB officers engaged with what were described as Ukrainian saboteurs. The FSB said it found 20 bombs, ammunition and mines in the area of the attack.
The agency said two more groups of saboteurs tried to force their way into Crimea late on Sunday with Ukrainian artillery and armored personnel carriers covering for their attempted raid. The FSB said one Russian soldier died in the incoming fire.
The FSB today described the incidents as "foiled terrorist attacks" and accused the Ukrainian military intelligence of plotting to target crucial parts of Crimea's infrastructure.
"Steps have been taken to destroy an agent network of the Chief Intelligence Department of the Ukrainian defense ministry in the Crimean peninsula," the statement said.
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The Ukrainian government reported during the weekend that Russia briefly closed its border crossings with mainland Ukraine, and social media users earlier this week posted photos and videos of dozens of armored vehicles on Crimean highways heading toward the de-facto border.
Ukrainian Security Service chief Yuri Tandit, in comments carried by the Interfax news agency on Wednesday, denied the FSB's report.
Earlier this month international monitors reported increased fighting between Ukrainian government troops and Russia-backed separatists in eastern Ukraine in a conflict which began in April 2014 following the Crimea referendum.