Russian armored carrier leads a column of military trucks as they leave the Ukrainian border around Kamensk-Shakhtinsky, Russia.

NATO Secretary General Anders Fogh Rasmussen said Friday the alliance has observed an alarming buildup of Russian ground and air forces in the vicinity of Ukraine.

"We have also seen transfers of large quantities of advanced weapons, including tanks, armored personnel carriers and artillery to separatist groups in eastern Ukraine," Rasmussen said in a statement.

Rasmussen said Russia continued to escalate the crisis in eastern Ukraine and that this could lead to further isolation of Moscow.

Read MoreMarkets wobble as Russian convoy enters Ukraine



He also condemned Russia for sending a "so-called humanitarian convoy" into Ukraine without the consent of the Ukrainian authorities and without any involvement of the International Committee of the Red Cross.

"It can only deepen the crisis in the region, which Russia itself has created and has continued to fuel," Rasmussen said in a statement.

He said it was a blatant breach of Russia's international commitments. Russia denies breaching international law.

Read More Ukraine accuses Russia of invasion as convoy enters



The United States, meanwhile, is "very concerned" about the movement of the convoy into Ukraine and is calling on Moscow to withdraw its equipment and personnel immediately, according to the Pentagon.

"This is a violation of Ukraine's sovereignty and territorial integrity by Russia," Rear Admiral John Kirby, the Pentagon press secretary, said in a Friday briefing. "Russia must remove its vehicles and its personnel from the territory of Ukraine immediately. Failure to do so will result in additional costs and isolation."

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