Ukrainian President Petro Poroshenko on Thursday instructed all military units near Crimea and the eastern Ukrainian Donbass region to be at the highest level of combat readiness.
Russia annexed Crimea from Ukraine in March of 2014, after Russian soldiers in unmarked uniforms took control of key government buildings.
Ukraine's president put his army on combat alert Thursday along the country's de-facto borders with Crimea and separatist rebels in the east as a war of words between Russian Federation and Ukraine threatened to heat up the largely frozen conflict over the Black Sea peninsula.
The Kremlin on Thursday said Putin chaired a Security Council session to discuss "additional measures" to ensure security at Crimea's de-facto border, the territorial waters around it and the Crimean airspace.
Mr Putin said the weekend attacks made French and German brokered talks with Mr Poroshenko at the G20 summit in China next month pointless.
Poroshenko rejected the Russian claims as "fantasy" and "a provocation".
The United Nations Security Council will meet Thursday to discuss rising tensions in Crimea Thursday at the request of Ukraine.
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Russian president Vladimir Putin said Kiev had "switched to terror tactics instead of searching for a peaceful settlement" to its conflict with Moscow, which after seizing Crimea fomented a separatist insurgency in eastern Ukraine that has killed some 10,000 people and displaced two million.
The spat has seen tensions between the countries rise to their highest level since Russian Federation annexed Crimea in 2014. While the separatists never allowed Ukrainian forces to regain control of the border with Russian Federation as agreed, Ukraine has also not adopted legislation to provide broad autonomy to these territories. Former Swedish Prime Minister Carl Bildt has expessed concern that Putin plans to invade Crimea again, saying that Putin has a pattern of launching military operations when the world's attention is focused on other matters, mainly during major sporting events. The FSB said Ukrainian forces attempted to cover the infiltration by directing heavy fire at the Russian side, killing two Russian servicemen. "Our special services have prevented a reconnaissance and sabotage group of the Ukrainian Defense Ministry from getting into Crimea".
"My biggest hope is that this discussion (in the council) will help the Russian Federation to understand that they can not really continue with this kind of behaviour", Yelchenko said.
The report said Russian forces spotted the "saboteurs" and while attempting to detain them, found "20 improvised explosive devices containing more than 40 kilograms of TNT equivalent, ammunition, fuses, antipersonnel and magnetic bombs, grenades and the Ukrainian armed forces' standard special weapons". The Ukrainian government did not immediately specify what further steps it was taking after putting forces on combat alert.
"The European Union condemns and doesn't recognise the illegal annexation of Crimea and Sevastopol by the Russian Federation", he said.
Pro-Russian separatists are fighting the Kiev government's forces in the eastern Ukraine region despite a fragile ceasefire.