NATO's top military commander has warned that a build–up of Russia troops on the Ukraine border suggests Moscow may be planning to invade more Ukrainian territory.

Russia was acting more like an adversary than a partner, NATO's Supreme Allied Commander Europe, US Air Force General Philip Breedlove said, and the 28-nation alliance should rethink the positioning and readiness of its forces in eastern Europe.

Russian troops, using armoured vehicles, automatic weapons and stun grenades, seized some of the last military facilities under Ukrainian control on Saturday in Crimea, the Black Sea peninsula that Russian President Vladimir Putin formally annexed the day before.

"The (Russian) force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," the NATO commander told an event held by the German Marshall Fund think-tank.

US President Barack Obama's deputy national security adviser Tony Blinken said the build-up might just be aimed at intimidating Ukraine's new pro-Western leaders but that Russia could invade the country's mainly Russian-speaking east.

"It's possible that they are preparing to move in," he told CNN.

A meeting of the G7 group of industrialised nations has been convened for tomorrow in Holland to allow leaders to discuss a response to Russia's actions.

Mr Obama will also meet Russian Foreign Minister Sergei Lavrov for bilateral talks.

Russia said it was complying with international agreements and had no plans to invade. It has called the soldiers who took over Ukrainian bases in Crimea "self defence forces".

The United States and the European Union have targeted some of Mr Putin's closest political and business allies with personal sanctions and have threatened broader economic sanctions if Putin's forces encroach on other eastern or southern parts of Ukraine with big Russian-speaking populations.

Germany, which has close trade ties with Russia, said the European Union was united in its readiness to impose sanctions on Russia if necessary, and that Moscow had the most to lose.

"None of us wants to escalate, but if Russia changes things unilaterally, then it must know that we won't accept it and that relations will be bad," Finance Minister Wolfgang Schaeuble told German television.

Ukrainian marine standards were still flying on Sunday alongside the Russian flag at the Crimean base of Ukraine's top military unit in Feodosia, but the Ukrainian troops were getting ready to leave after the Russian military takeover.

"Our only issue is that we want to leave this place with honour, weapons and vehicles," one Ukrainian soldier said.