KIEV, Ukraine — Tanks and other military vehicles pouring over the border from Russia into eastern Ukraine. Nightly artillery battles in the region’s biggest city, Donetsk, and reports of fighting around another regional capital. And now, sightings of the “green men,” professional soldiers in green uniforms without insignia, the same type of forces that carried out the invasion of Crimea last spring.

A senior NATO official confirmed on Wednesday what Ukrainian military officials and monitors from the Organization for Security and Cooperation in Europe have been saying for days now: Russian troops and military equipment are crossing the border into Ukraine, seemingly preparing for renewed military action, though what exactly remains unclear.

The statement drew stern and dismissive denials from Moscow, which for months has denied any military intervention in eastern Ukraine, though it has acknowledged publicly that Russian “volunteers” have crossed into Ukraine to support the pro-Russia separatists.

In light of the military buildup, Western officials finally seemed ready to acknowledge that a cease-fire agreement signed in September had fallen apart, and that the threat to peace in Europe posed by the Ukraine crisis had returned in a possibly more virulent form.