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.
"I have ordered all forces in the regions of the administrative cordon with Crimea and all contact lines in the Donbass to maintain increased combat readiness," Poroshenko said in a Twitter post on Thursday.
Earlier in the day, Russian President Vladimir Putin held an urgent meeting with his security council to discuss increased security measures in Crimea as Moscow claims it thwarted an attempted attack by Ukrainian military forces on critical infrastructure in the peninsula.
Putin discussed measures to secure Crimea's land border with Ukraine as well as its surrounding waters and airspace, the Kremlin said in a statement. The meeting was attended by Russia's prime minister, defence minister and foreign minister.
Russia occupied and annexed Ukraine's southern Crimea region in early 2014, in response to Kiev ousting its pro-Russian president amid mass protests calling for closer ties with the West.
Soon after the annexation of Crimea, a pro-Russian separatist rebellion erupted in Ukraine's two easternmost regions. Ukraine has accused Russia of fuelling the conflict with weapons and soldiers for the separatists, allegations Moscow has denied.
In New York, Ukraine requested an urgent meeting of the UN Security Council to discuss Russia's "latest provocations in Crimea."
Volodymyr Yelchenko, Ukrainian ambassador to the UN, thanked council members that "without any questions" reaffirmed Ukraine's territorial integrity including Crimea during the consultations held behind closed doors.
Yelchenko said he hoped that discussions at the council could lower the current level of hostilities, noting that Russia has amassed a total of 40,000 troops in Crimea, eastern Ukraine and along the border.
"These numbers may reflect some very bad intentions," Yelchenko said. "This is the last thing we would like to happen."
Vitaly Churkin, Russian ambassador to the UN, said he hoped Ukraine was "prudent enough" to do everything to de-escalate the situation, calling Ukraine's alleged thwarted attack "a clear act of sabotage and terrorism."
"Don't get engaged in acts of sabotage and terrorism - this is my good advice to my Ukrainian friend," Churkin said.
A NATO official noted that Russia had "not provided any tangible evidence for its accusations against Ukraine," while a spokeswoman for the European Union's diplomatic service said there had been "no independent confirmation" of the claims.
The Organization for Security and Cooperation in Europe (OSCE), which monitors the conflict in eastern Ukraine, has no access to the peninsula, the EU spokeswoman noted.
Russia's Federal Security Service (FSB), a successor agency of the Soviet KGB, said Wednesday that it had detained several men who were in possession of 20 explosive devices and are believed to have been working for Ukraine's military intelligence agency.
Two Russians - a soldier and a member of the FSB - were killed in related clashes with the Ukrainian military forces, the FSB said in a statement.
Poroshenko responded to the claim by calling it a "pretext for more military threats against Ukraine."
"Russian accusations that Ukraine has launched terror attacks in occupied Crimea are as cynical and insane as claims that there are no Russian troops in eastern Ukraine," Poroshenko said in a statement.
The claims follow a recent surge in fighting in eastern Ukraine, with last month reported as the deadliest in more than a year.
NATO is monitoring the heightened tension between Moscow and Kiev with concern, an alliance official said.
The unnamed official said that a Russian military build-up seen on the Crimea-Ukraine border in recent days "is not helpful for easing tensions," speaking on condition of anonymity. He called on Russia to "work for calm and de-escalation."
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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 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.