Ukraine's Defense Ministry warned on Wednesday that Russia has moved some 500 attack planes, 340 combat helicopters, and 25 "battalion-tactical groups" to bases near the Ukrainian-Russian border. Vadym Skibitsky, a spokesman for the Intelligence Directorate, added that these forces are combat-ready with experience from Syria.

Russian President Vladimir Putin earlier in the day laid the blame for a standoff with Ukrainian vessels in the Black Sea on the Ukrainian president and his desire to get re-elected.

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It is the first time that Putin commented on the incident near Russian-occupied Crimea on Sunday that raised the specter of a full-blown conflict between the two neighbors. Earlier Wednesday, Russia said it planned to deploy more of its advanced S-400 surface-to-air missile systems to Crimea as tensions rose with Ukraine.

Putin said in a televised speech on Wednesday afternoon that the incident was entirely provoked by the Ukrainian vessels, which refused to communicate with Russian border guards. Putin laid the blame on Ukrainian President Petro Poroshenko, saying that he ordered the navy to provoke the standoff with the sole purpose of scoring political points and getting re-elected next year.

Putin also said the Ukrainian vessels violated the territorial waters off southern Russia — which is the internationally recognized border. This appears to run counter to the claims of the Ukrainian government, which said the ships were approaching from another direction and were firmly in international waters.

>> Read more: Russian aggression against Ukraine may not start WWIII, but hints at troubled waters for Putin ■ Russia vs. Ukraine: What's behind the Black Sea naval standoff pushing the countries to brink of war

Open gallery view Ukrainian soldier patrols aboard military boat "Dondass" moored in Mariupol, Sea of Azov port on November 27, 2018 Credit: AFP

Meanwhile, the U.S. called on European allies to consider further sanctions against Russia over the situation in Ukraine, the U.S. Special Representative for Ukraine Negotiations said on Wednesday.

"We certainly encourage our European allies to look harder at what additional sanctions could be implemented," Kurt Volker said in Berlin. "I've seen at this point that is not something Germany and France have said they're considering but these things, I think, can develop over time."

Several senior European politicians on Tuesday raised the possibility of new sanctions against Russia to punish it for capturing three Ukrainian vessels at sea, an incident the West fears could ignite a wider conflict.