World leaders are to meet for emergency G7 talks on Russia's annexation of Crimea, after NATO gave warning that the Russian troop build-up in Crimea posed a wider threat to other post-Soviet states.

Monday's hastily arranged talks, on the sidelines of a nuclear security summit in the Netherlands, will not involve Russia - a symbolic snub to Moscow, the host of a G8 summit this year in Sochi which now looks unlikely to take place.

The leaders of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States will discuss the crisis as Russian troops continue to tighten their grip on Crimea, having taken over a total of 189 sites used by the Ukraine military,.

NATO's top military commander said on Sunday that Russia had amassed a force on Ukraine's eastern border large enough to pose a threat to other post-Soviet republics.

"The force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," said General Philip Breedlove, the supreme allied commander in Europe.

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

On Sunday, Ukraine's national security chief, Andriy Parubiy, told a mass rally in Kiev: "The aim of Putin is not Crimea but all of Ukraine.... His troops massed at the border are ready to attack at any moment."

The EU and the US have imposed a number of visa bans and asset freezes on some members of the inner circle of the Russian President, Vladimir Putin, and are considering further measures.

The German finance minister, Wolfgang Schaeuble, said on Sunday that the EU was united in its readiness to impose economic sanctions on Moscow should the crisis in Crimea escalate.

"Russia has a lot more to lose in the medium term than the West, than Europe or the United States," he said, adding that the aim was to uphold international law and it was "of secondary importance whether there is an economic or financial cost".

Anatoly Antonov, Russia's defense minister, said on Sunday that "Russian armed forces do not undertake any undeclared military activity that would threaten the security of neighboring countries".

One Ukrainian serviceman was killed and two others wounded since Russian forces started seizing control of military facilities in Crimea last week.

Ukraine's Defense Ministry said on Sunday that most of its bases in Crimea were now occupied by Russian troops and flew Russia's tricolor flag.

The Russian defense minister, Sergei Shoigu, ordered the Black Sea Fleet commander on Friday to ensure Ukrainian servicemen based in Crimea were allowed to leave for mainland Ukraine.

As of March 21, fewer than 2,000 of 18,000 Ukraine troops in Crimea had expressed a wish to do so, according to Ukraine's government.

PHOTO CAPTION

A general view of the coast on March 22, 2014 in Yalta, Crimea, Ukraine.

Aljazeera