( 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the Russian military buildup and rebutted Russian claims that other satellite photos released a day earlier were outdated .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 .
NATO 's secretary general says Russia should de-escalate border buildup . NATO releases more photos that it says shows buildup to rebut Russia 's denials . Destroyer USS Donald Cook enters the Black Sea . Russia has no plans to annex southeastern Ukraine , the country 's foreign minister says .