The death of Russian servicemen in armed clashes on the border between the annexed Crimean peninsula and the rest of Ukraine over the weekend will have consequences, Russia has warned.
The Russian foreign ministry said in a statement on Thursday that attempts to destabilise the situation in Crimea would fail, referring to the alleged attack from the Ukrainian side.
Russian President Vladimir Putin held a meeting with security chiefs to discuss "additional measures for ensuring security for citizens and essential infrastructure in Crimea", the Kremlin said in a statement.
"Scenarios were carefully considered for anti-terrorist security measures at the land border, in the waters and in the airspace of Crimea," it said.
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Two residents of Crimea told the AFP news agency there had been an unexplained build-up of Russian military hardware in the area over the past few weeks.
Ukraine has denied the clashes and deaths took place, but placed its forces around Crimea on high alert as the tensions soared.
A senior Ukrainian security official told AFP that Moscow's claims were a "crude Russian provocation" and that Kiev was "getting ready for anything", including an invasion by Moscow.
Ukrainian President Petro Poroshenko said that he had asked his foreign minister to arrange telephone conversations over the issue with Putin as well as the leaders of Germany and France, US Vice President Joe Biden and European Council President Donald Tusk.
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The UN Security Council was to discuss the growing tensions later on Thursday at the request of Ukraine, a non-permanent council member.
NATO is concerned about the heightened tension between Moscow and Kiev, an alliance official told the DPA news agency on condition of anonymity.
"We are also deeply concerned by the recent upsurge in violence in eastern Ukraine, and the increase in ceasefire violations along the line of contact, primarily by the Russian-backed militants," the official said.
He called on Russia to "work for calm and de-escalation", while urging all parties back to the negotiating table to achieve a peaceful settlement.
Moscow and Kiev have been locked in a bitter dispute since the Kremlin seized Crimea in March 2014 after Ukraine's Russian-backed President Viktor Yanukovych was ousted.