US President Barack Obama has arrived in the Netherlands for a G7 summit on the Ukraine crisis and nuclear talks.

Russian troops have seized a Ukrainian naval base in Crimea amid growing fears of a fresh Russian incursion into Ukraine.

The base, in Feodosia, was one of the last few military facilities flying a Ukrainian flag after Russia's annexation of Crimea.

Over the weekend Russian troops have been violently flushing out remaining pockets of Ukrainian military influence in Crimea.

As G7 leaders are prepare to discuss the crisis at a summit, Nato's top commander in Europe has warned that Russia's military force massing on Ukraine's border was "very, very sizeable and very, very ready".

General Philip Breedlove said he was worried the Russian military could make a move for Moldova's breakaway Transdniestria region.

"There is absolutely sufficient force postured on the eastern border of Ukraine to run to Transdniestria if the decision was made to do that and that is very worrisome," he said.

Transdniestria, a narrow strip of land to Ukraine's southwest, already has a Russian military presence and most people there favour a union with the country.

White House deputy national security adviser Tony Blinken echoed the suggestion that President Putin could be plotting further action.

"It's deeply concerning to see the Russian troop build-up on the border,” he told CNN.

"It creates the potential for incidents, for instability. It's likely that what they're trying to do is intimidate the Ukrainians. It's possible that they're preparing to move in."

Russian President Vladimir Putin last week signed papers making Crimea part of Russia, saying it was complying with international agreements and had no plans to invade.

It has also called the soldiers who took over Ukrainian bases in Crimea "self-defence forces".

Most experts agree there is no way of winning back Crimea from Russia.