NATO is concerned about heightened tension between Moscow and Kiev, an alliance official says on condition of anonymity, adding that recent Russian military activity in Crimea "is not helpful."
"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 says.
He calls on Russia to "work for calm and de-escalation," while urging all parties back to the negotiating table to achieve a peaceful settlement.
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Ukrainian President Petro Poroshenko has ordered the military to be on combat alert on the de facto border with Crimea and in the country's two easternmost regions amid heightened tensions with Russia.
Russian claims that Ukrainian military forces attempted a terrorist attack in Crimea are unverified, an EU foreign policy spokeswoman says, noting that the Organization for Security and Co-Operation in Europe has no access to the occupied peninsula.
Russian President Vladimir Putin met with the country's security council on Thursday to discuss further security measures to secure Crimea's land border with Ukraine as well as its surrounding waters and airspace.
Russian President Vladimir Putin on Wednesday vowed to take steps to ensure safety in Crimea, hours after Russian authorities accused Ukraine of attempting to attack critical infrastructure in the region with two Russians killed in related clashes.