(CNN) -- NATO Secretary General Anders Fogh Rasmussen condemned Friday what he called a "serious escalation of Russian aggression" in Ukraine and said Moscow is carrying out direct military operations there. Accusing Moscow of "hollow denials," Rasmussen said Russian troops had illegally crossed the border into Ukraine and that this was not an isolated action but part of a campaign to "destabilize Ukraine as a sovereign nation." Rasmussen, who spoke after a meeting of NATO ambassadors in Brussels, Belgium, said Russia's action was a blatant violation of Ukraine's sovereignty and "defies all diplomatic efforts for a peaceful solution."