NATO Secretary-General Anders Fogh Rasmussen has vowed support for Ukraine in the face of Russian "aggression" and called on Moscow to pull back its troops from the border.
Rasmussen, speaking on a visit to Kyiv on August 7, called on Russia "to step back from the brink, step back from the border, and not use peacekeeping as an excuse for war-making."
He said that Russia, instead of de-escalating the conflict, continued to destabilize Ukraine, and that its support for pro-Russian separatists fighting government forces in the east was growing "in scale and sophistication."
NATO said on August 6 that a Russian buildup near the Ukrainian border amounted to some 20,000 troops.
The alliance said Moscow could use the excuse of a humanitarian or peacekeeping mission to send them into Ukraine.
Speaking after talks with Ukrainian leaders, Rasmussen said Ukraine and NATO were stepping up their military cooperation, with Ukrainian President Petro Poroshenko set to attend a NATO summit in September.
Rasmussen also said the alliance was planning more joint exercises with Kyiv, as well as more shared training and education.
He also said NATO was considering "long-term assistance to modernize the Ukrainian armed forces and the Ukrainian security sector."
Russia has long been wary of closer ties between Ukraine and NATO.
Rasmussen said that "it is the right of every country to choose its own foreign policy without foreign interference. NATO fully respects that right but today Ukraine's freedom and future are under attack."