MOSCOW: President Vladimir Putin has accused Ukraine of resorting to "terror" after Russia announced that two servicemen were killed during a supposed clash in Crimea.
The incident has triggered the worst escalation of tensions between the two countries for several months, amid reports of Russian troop movements.
The Kremlin's security agencies said that two Russians were killed as they thwarted Ukrainian commando raids into Crimea over the weekend.
But Ukrainian officials denied that any such incident had taken place and accused Mr Putin of seeking a pretext for war. Russia occupied Crimea in 2014 before invading eastern Ukraine, triggering a war that has claimed 9,000 lives and forced 1.7?million people from their homes.
"From the Russian side there were losses - two soldiers killed. We obviously will not let such things slide by," Mr Putin told journalists in Moscow.
He accused Ukraine of an attempt to "provoke conflict", adding that he would not meet Petro Poroshenko, the president of Ukraine, at the G20 summit in China next month.
Russia's Federal Security Service (FSB), which controls the country's border guards, said that one of its officers and one soldier were killed in separate incursions by Ukrainian forces on Sunday and Monday.
"Steps have been taken to destroy an agent network of the Chief Intelligence Department of the Ukrainian defence ministry in the Crimean peninsula," said an FSB statement, adding that several Ukrainian servicemen had been captured during the incidents.
But a Ukrainian military spokesman countered the claims by saying this was "fake information".
Dmytro Kuleba, a spokesman for Ukraine's foreign ministry, said: "Putin wants more war. Russia escalates, desperately looks for casus belli against Ukraine, [and] tests West's reaction."
Since occupying Crimea, Russia has assembled a large military presence on the peninsular. It is building a bridge across the Kerch strait to connect Crimea to the Russian mainland. The region has been relatively unaffected by the war between government forces and Russian-backed rebels, along with regular Russian forces, in eastern Ukraine. But tensions have mounted amid reports of a major build-up of Russian troops.
Russian border guards briefly closed road crossings from Crimea to the rest of Ukraine on Sunday. On the same day, Crimean Tatar activists reported seeing large numbers of military vehicles moving into the border towns of Dzhankoy and Armyansk.
The FSB said that its personnel had defeated a "terrorist" group and seized an arms cache including 20 improvised explosive devices. The agency claimed that Ukrainian forces had tried to "break through" twice more on Sunday night and Monday morning, killing a Russian soldier.