NATO's top military commander has said he is "very concerned" that Russia's military build-up in the annexed Crimean region could be used as a launchpad for attacks across the whole Black Sea region.

US General Philip Breedlove's comments late on Wednesday came amid fears in Kiev that Russian-backed rebels will try to grab more land in eastern Ukraine to establish a land corridor to Crimea, which was annexed by Russia in March.

"We are very concerned with the militarisation of Crimea," Breedlove said, following meetings with Ukraine's top political and military leaders in Kiev.

"The capabilities that are being installed in Crimea ... are able to exert influence over the entire Black Sea," he said, highlighting the influx of cruise missiles and surface-to-air rockets.

Russia's Defence Ministry said Wednesday that it had deployed a batch of 14 military jets to Crimea as part of a squadron of 30 that will be stationed on the peninsula.

Also on Wednesday, an OSCE Special Monitoring Mission (SMM) in eastern Ukraine was reportedly attacked with rocket-propelled grenades and anti-aircraft ammunition.

"These shots appear to have been fired from separatist-controlled territory," the US Department of State said n a statement on Thursday.

"Any attacks on, or threats to, OSCE monitors or equipment are unacceptable. We call on all parties to refrain from any actions that endanger the safety of the OSCE mission in Ukraine."



Meanwhile, deadly clashes between government forces and the separatists rumbled on, with Ukraine's military saying two soldiers were killed in the past 24 hours.

Ukraine's new government this week stated its desire to move towards NATO membership, triggering further ire from Russia which strongly opposes the expansion of Western institutions in what it considers its backyard.

The Ukrainian public has previously not been keen on NATO membership but there has been a dramatic shift in opinion since Russia's involvement in the separatist uprising, that has cost the lives of more than 4,300 people since April.