NATO's top military commander said he was "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.

U.S. General Philip Breedlove's comments 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 militarization 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 defense 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.

Breedlove also repeated accusations that Russian troops were inside east Ukraine "training, equipping, giving backbone" to rebel forces, as Ukraine's military said several more columns of military hardware and troops were spotted crossing the border from Russia on Tuesday.

Moscow fiercely rejects any claims that it is involved in the fighting in east Ukraine but gives open political backing to the rebels, who now boast the heavy weaponry of a regular army.

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.

A recent poll found 51 percent of Ukrainians now backed membership in the alliance, up from 20 percent a year ago.

Joining NATO remains a remote possibility, however, since Ukraine lacks secure borders and current members are wary of further provoking Russia.

Its ill-equipped army would also need a radical overhaul to meet membership criteria, although Breedlove said the ongoing training of Ukraine's military was bringing them closer to "NATO tactics, techniques and procedures."

Ukraine has unsuccessfully been pushing the U.S. to send weapons to battle the separatist forces but Washington has only delivered non-lethal military aid, such as night-vision goggles and radio equipment.

On the ground in east Ukraine, in the beleaguered rebel-held city of Donetsk, life for ordinary people grew harder as cash machines dried up after Ukraine's central bank gave the order Tuesday to shut down the banking system in areas controlled by the rebels.

An AFP journalist saw dozens of people queue up outside banks in the biting cold in the vain hope of withdrawing money.

"We received the order to close the establishment yesterday. There are no more money transfers here," said Valery, a worker at state-run Oschadbank.

The move is the latest step by Kiev to cut off the separatist territories following a Nov. 14 decree by President Petro Poroshenko to halt all state services including schools and hospitals in the conflict zone.

In a sign that the measures could be starting to bite, Ukraine's military said the number of people leaving the insurgent area over the past day increased 10-fold to 5,677.

Russia on Wednesday poured cold water over a request from the pro-Moscow rebels in east Ukraine for a UN peacekeeping force.

Asked about the appeal, Russia's Ambassador to the United Nations Vitaly Churkin said: "I think it's unlikely."

Leaders of the self-proclaimed Donetsk People's Republic called Tuesday for "the intervention of a peacekeeping force that includes Russian representatives to control the implementation of existing agreements and help resolve the humanitarian and social situation."

More than 4,300 people have been killed and nearly half a million displaced in the fighting that has raised East-West tensions to their highest level since the end of the Cold War.

Some one million people have so far fled the fighting in east Ukraine, with the majority heading over the border into Russia.