On Friday, Russian Prime Minister Dmitry Medvedev said Moscow could formally sever diplomatic ties with Kyiv after reported clashes between Russian forces and alleged Ukrainian "saboteurs" in the annexed Crimean Peninsula.

"I would not like this to end this way. However, if there is no other option to influence the situation, the president can probably make such a decision," the premier said in comments carried by Russia's state-owned TASS news agency.

"This act of sabotage is an act of crime in its essence, despite the fact that its plot failed, and it should be properly investigated while the persons implicated in it as well as organizers should bear responsibility - criminal responsibility," Medvedev said. He was referring to Russian allegations that Ukrainian special forces sought to sabotage key Crimean installations over the weekend, claims denied by Kyiv.

In response to accusations that Kyiv had attempted to undermine Russia's self-declared sovereignty in Crimea, President Petro Poroshenko placed Ukraine's military on "high-alert level" and deployed tanks to its frontier with Crimea. Russia has also amassed forces near the border, according to news agencies.

Poroshenko said Moscow's claims were "fantasies" aimed at providing Russia with a "pretext for the next military threats."

The S-400 "Triumph" missile defense system is considered Russia's latest anti-aircraft setup

Also on Friday, Russia's southern military district reported that troops in Crimea had "received the modern S-400 'Triumph' air defense system."

'Only words'?

Meanwhile, the UN Security Council held an overnight closed-door meeting in New York at Kyiv's request to discuss the latest developments in Crimea.

Ukraine's UN Ambassador Volodymyr Yelchenko called for Russia to submit proof of the allegations that Ukrainian security officers were conducting an operation in Crimea.

"If it is happened in reality, where are the proofs? Statements, pictures, photos, videos, whatever," Yelchenko said. "They are only words."

Relations between Moscow and Kyiv have nose-dived since Ukraine's Kremlin-friendly government was ousted, prompting Moscow to intervene in Crimea in 2014. The move was subsequently reinforced by a public referendum where voters expressed a desire to join the Russian Federation. However, the vote was condemned by the UN, Germany, the US and other Western allies.

The Crimean city of Sevastopol is one of Russia's most strategically important ports and naval bases; Ukraine and Russia eventually agreed to share the facilities after the dissolution of the Soviet Union.

After President Viktor Yanukovych fell in 2014, fighting also broke out in Ukraine's eastern regions, which had been the stronghold of his elected government. Attempts to broker a ceasefire, spearheaded by the Organization for Security and Cooperation in Europe and by Germany, have yielded only moderate success, while fighting has intensified in the region in recent weeks.

More than 9,500 people have been killed in the Ukrainian conflict, with more than 500,000 children affected, according to UN figures.

Watch video 00:47 UN Security Council meets over Crimea | DW

ls/msh (Reuters, AP, AFP)