NATO's top military commander has warned that Russian militarisation of the annexed Crimea Peninsula could be used to exert control over the whole Black Sea.
NATO's Supreme Allied Commander for Europe, US General Philip Breedlove, has accused Russia of keeping forces in Ukraine to help separatists, and has expressed fears that Russia may be marshalling resources to push for more territory.
General Breedlove spoke during a brief visit to Kiev, where he met with top officials to discuss continued NATO assistance in Ukraine.
"The capabilities that are being installed in Crimea will bring an effect on almost the entire Black Sea," General Breedlove said.
"There's a great force there that can be exerted if it's required. Inside Ukraine we still see a large number of Russians who are involved primarily in training, advising, assisting and helping the forces of the Russian-backed forces in the East."
General Breedlove was set to meet with Ukrainian president Petro Poroshenko, as Kiev stirred further Russian ire by stating it hoped to join the Western security alliance.
The capabilities that are being installed in Crimea will bring an effect on almost the entire Black Sea.
US General Philip Breedlove, NATO Supreme Allied Commander Europe.
Mr Poroshenko mooted an eventual referendum on joining NATO, and Ukraine's new pro-Western government included a desire for membership in its official program.
Russia is strongly opposed to the expansion of Western institutions in what it considers to be its backyard.
Russian deputy foreign minister Sergey Ryabkov said Ukraine's efforts to join NATO would "lead only to further complication of the situation".
"This will not increase security for Ukraine, it will not improve the life of the Ukrainian people," he told reporters.
The Ukrainian public was never keen on NATO membership in the past but there has been a dramatic shift in opinion since Russia annexed Crimea in March, with 51 per cent backing NATO membership in a recent poll.
Kiev argued that it needed to join NATO in order to stave off alleged aggression from Moscow, which it accused of stirring the conflict in the east that has killed over 4,300 people since April.
Moscow routinely denied it played any direct role in the conflict in Ukraine.