NATO's top commander says Russian forces on the eastern Ukrainian border are very sizeable and very ready.

General Philip Breedlove suggested the Russian forces were large enough to mount a full scale invasion.

"The [Russian] force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," Breedlove said.

One potential flashpoint is Trans-Dniester, the self-proclaimed independent region, still formally recognised by the international community as part of Moldova. Trans-Dniester is a narrow strip of land which runs between Moldova and Ukraine.

Breedlove warned that this region could become a central trigger to further Russian action.

"There is absolutely sufficient force postured on the eastern border of Ukraine to run to Trans-Dniester if the decision was made to do that and that is very worrisome.

"Russia is acting much more like an adversary than a partner."

Meanwhile, Russia says its flag is now flying over 189 Ukrainian military installations in the Crimea.

It comes two days after Russian President Vladimir Putin signed laws annexing the region.

Ukraine's Ambassador to the UK, Volodymyr Khandogiy says Russia's actions are a worrying development.