(CNN) -- NATO's chief urged Russia on Friday to pull back its troops from its border with Ukraine.
Russia should contribute "to a de-escalation of the situation" and engage in a direct dialogue with the Ukrainian government, NATO Secretary General Anders Fogh Rasmussen said during a visit to Sofia, Bulgaria.
NATO is "not discussing military actions" but is focused on protecting its allies, and it "will take all steps necessary to make sure that this collective defense is effective," Rasmussen said.
Meanwhile, the organization is reinforcing support to its allies, "from the Baltic to the Black Sea," to deal with the instability created by Russia, Rasmussen said via Twitter.
NATO released additional satellite images Friday purportedly showing the Russian military buildup and rebutted Russian claims that other satellite photos released a day earlier were outdated. NATO said the photos were recent, gathered between late March and earlier this month.
"It is clear that the military buildup of forces occurred in early March 2014," NATO said.
Meanwhile, the destroyer USS Donald Cook has entered the Black Sea on a mission to demonstrate the U.S. "commitment to our allies and to enhance security readiness in the region," Navy spokesman Lt. Shawn P. Eklund said Friday.
Russia has called the U.S. military move, however, part of a systemic NATO buildup in the Black Sea.
An international crisis arose in Ukraine after Russia annexed its Crimean Peninsula in March under a pact signed by Russian President Vladimir Putin with the Prime Minister of Crimea and the mayor of the city of Sevastopol. Earlier, in a Crimean referendum, 96.7% of ballots favored Crimea leaving Ukraine and being annexed by Russia.
The United States now estimates there are up to 40,000 Russian troops near the border, based on classified and commercial satellite imagery.
Russia has no intention to additionally annex the southeastern regions of Ukraine, Foreign Minister Sergey Lavrov said Friday in a televised interview on Russian state television, the state-run ITAR-Tass news agency reported.
"This would run counter to the vital interests of Russia," Lavrov said, while adding that there is no need to deploy peacekeepers in Ukraine nor any need for United Nations Security Council decisions on the matter.
Lavrov went on to say that there are no Russian troops or Russian secret services agents in southeastern Ukraine.
CNN's Michael Martinez and Pierre Meilhan contributed to this report.