Nato has called on Russia to "withdraw its forces to its bases, and to refrain from any interference elsewhere in Ukraine".
We call upon the Russian Federation to honor its international commitments, including those set out... the legal framework regulating the presence of the Russian Black Sea Fleet, to withdraw its forces to its bases, and to refrain from any interference elsewhere in Ukraine.
We urge both parties to immediately seek a peaceful resolution through bilateral dialogue, with international facilitation, as appropriate, and through the dispatch of international observers under the auspices of the United Nations Security Council or the Organization for Security and Cooperation in Europe (OSCE).
A Nato statement said the legal framework was agreed in Budapest Memorandum of 1994, the Treaty on Friendship and Cooperation between Russia and Ukraine of 1997.
Pro-Russia activists recaptured administrative headquarters in the Ukrainian city of Donetsk and flew the Russian flag from its roof.
ITV News' Europe Correspondent describes the tense atmosphere surrounds a central government building in Donetsk .
New footage obtained by ITV News shows Russian troops firing warning shots as Ukrainian soldiers approached during a tense stand-off today.