Russia is "more and more" isolated over its actions in Ukraine, NATO said on Thursday, while calling for beefed up alliance deployments to defend its eastern members.

"Nobody wants to isolate Russia but Russia is isolating itself internationally more and more every day," the alliance's chief Anders Fogh Rasmussen said Friday, as tensions remain high over the annexation of Crimea.

The warning shot came a day after NATO released satellite pictures showing up to 40,000 Russian troops massed along the Ukraine border.

"I call on Russia to take concrete steps to regain the trust of the international community, pull back its troops from Ukraine's borders and enter a sincere dialogue respecting Ukraine's sovereignty," the NATO chief said during a visit to Sofia.

The alliance described the troop movements Thursday as a "real threat" to Kiev.

Amid fears that the crisis could spread to NATO's eastern European members, the alliance has enhanced air policing over the Baltic states, deployed reconnaissance planes over Poland and Romania and overseen an increased naval presence in the Black Sea, Rasmussen said.

"But I think we need to take more steps and we will -- based on recommendations from our military authorities -- discuss this in the coming days and weeks.

"These considerations might include an update and development of our defence plans, enhanced exercises and also appropriate deployments," he said.

"We are not discussing military options but NATO is focused on ensuring defence and protection of all our allies and we will take all steps necessary to make sure that this collective defence is effective," Rasmussen went on.

"Let there be no mistake -- those steps are entirely in line with our international commitments and with our agreements with Russia, which Russia itself continues to blatantly breach," he added, condemning Moscow's "illegitimate actions".

After Moscow's takeover of Crimea last month, tensions have now spread to eastern Ukraine, where militants have stormed a series of strategic government buildings and demanded that Moscow send its troops for support.