A Russian security agent and a soldier have been killed fending off what Moscow described as a series of attempted terror attacks by the Ukrainian military intelligence in Crimea.
Russian President Vladimir Putin condemned what he called a "stupid criminal action" by the Ukrainian authorities and vowed to take additional steps to ensure security of Crimea. He also strongly urged the West to warn Kiev against "resorting to terror instead of searching for a peaceful settlement".
Ukrainian officials have denied the claims.
Russia annexed the Black Sea peninsula in March 2014 following a hastily called referendum. The ensuing conflict in eastern Ukraine has killed more than 9,500, and fighting there between Ukrainian troops and Russia-backed separatists has continued despite a 2015 ceasefire.
The Federal Security Service, known under its Russian acronym the FSB, said its officer was killed over the weekend near Armyansk within a few miles of the de-facto border between Crimea and Ukraine when FSB officers engaged in a gun battle with a group of "saboteurs" from Ukraine.
The FSB said the intruders carried an arsenal of bombs, ammunition and mines.
The agency said two more groups of saboteurs tried to force their way into Crimea early on Monday, supported by Ukrainian artillery. It said one Russian soldier died in that clash.
The FSB said it also busted what it called a network of agents of Ukrainian military intelligence in Crimea and detained several people, including a Ukrainian citizen identified as Yevgeniy Panov whom it described as a Ukrainian military intelligence officer.
The agency claimed the Ukrainian intelligence operation had sought to destabilise the situation in Crimea ahead of Russia's parliamentary election next month.
An aide to the Ukrainian Security Service chief, Yuri Tandit, denied the Russian claims in comments carried by the Interfax news agency.
But Mr Putin, speaking at a news conference in Moscow, accused the Ukrainian leadership of engaging in "terror" instead of discussing a peace settlement in eastern Ukraine. Because of that, he said, it makes no sense to discuss the Ukrainian peace settlement with leaders of Ukraine, France and Germany at the sidelines of the Group of 20 meeting in China next month as earlier planned.
The Ukrainian government said over 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 armoured vehicles on Crimean road heading toward the de-facto border.