Russia's main domestic security agency said Wednesday that one of its agents and an army soldier were killed while fending off what it described as a series of attempted terror attacks by Ukrainian "saboteurs" in Crimea, a claim Ukrainian officials denied.

Russia's President Vladimir Putin condemned what he described as 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 President Petro Poroshenko rejected the Russian claims as "fantasy" and "provocation," saying in a statement that his government would use only political and diplomatic means to restore its sovereignty over Crimea.

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 cease-fire.

The Federal Security Service, known under its Russian acronym FSB, said in Wednesday's statement that 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 tried to force their way into Crimea early Monday, supported by Ukrainian artillery and armor. It said one Russian army 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 that the Ukrainian intelligence operation had sought to destabilize the situation in Crimea ahead of Russia's parliamentary elections set for next month.

An aide to the Ukrainian Security Service chief, Yuri Tandit, denied the Russian claims Wednesday in comments carried by the Interfax news agency.

But Putin, speaking at a news conference in Moscow, accused the Ukrainian leadership of engaging in "terror" instead of discussing 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 armored vehicles on Crimean highways heading toward the de-facto border.