Ukrainian Defense Minister Oleksii Reznikov has called for global unity to prevent a new Russian "escalation" against Ukraine during a visit to the Pentagon where he met with his American counterpart to discuss Russia.
The Thursday visit came as both Kiev and Washington warned what they described as unusual Russian military activity.
The US military has thus far stopped short of describing Russia's build-up as a preparation for attack.
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On Wednesday, US Defense Secretary Lloyd Austin conceded that "we're not sure exactly what Mr. Putin is up to." Reznikov met Austin at the Pentagon and cautioned about the consequences of any Russian military moves.
"This possible escalation will certainly have catastrophic consequences for the whole of Europe," Reznikov said, according to a Pentagon transcript of a portion of his remarks.
"Whether it will take place depends to a large extent on the unity and determination of the civilised world to stop the aggression."
Border build-up
Russian President Vladimir Putin on Thursday accused NATO and Western countries of escalating the situation in the Black Sea.
Speaking at an expanded meeting of the Foreign Ministry Board in Moscow, Putin said NATO and the West go "beyond limits" conducting "provocative military exercises," and sending strategic bombers with "very serious weapons" to fly at a distance of only 20 kilometers from the Russian border.
There have been several waves of NATO eastward expansion, anti-missile defense systems have been deployed right next to Russian borders with Romania and Poland, the president added.
"These (anti-missile systems) can easily be put to offensive use with the Mk-41 launchers there; replacing the software takes only minutes," he noted.
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Russia repeatedly voiced its concerns but until recently the West has had a "superficial approach" to such warnings about red lines, he said.
"However, our recent warnings are flaring up and produce a certain effect. A known tension has arisen there after all," he said. It is imperative to push for serious long-term guarantees that ensure Russia's security along its western borders, Putin stressed.
"No matter how difficult it may be, we need to work on this, and I want you to keep that in mind," he said.