DONETSK, Ukraine — Unexpectedly, at the height of the Ukrainian winter, war has exploded anew on a half-dozen battered fronts across eastern Ukraine, accompanied by increasing evidence that Russian troops and Russian equipment have been pouring into the region again.

A shaky cease-fire has all but vanished, with rebel leaders vowing fresh attacks. Civilians are being hit by deadly mortars at bus stops. Tanks are rumbling down snowy roads in rebel-held areas with soldiers in unmarked green uniforms sitting on their turrets, waving at bystanders — a disquieting echo of the “little green men” whose appearance in Crimea opened this stubborn conflict in the spring.

The renewed fighting has dashed any hopes of reinvigorating a cease-fire signed in September and honored more in name than in fact since then. It has also put to rest the notion that Russia’s president, Vladimir V. Putin, would be so staggered by the twin blows of Western sanctions and a collapse in oil prices that he would forsake the separatists in order to foster better relations with the West.

Instead, blaming the upsurge in violence on the Ukrainians and the rise in civilian deaths on “those who issue such criminal orders,” as he did on Friday in Moscow, Mr. Putin is apparently doubling down, rather than backing down, in a conflict that is now the bloodiest in Europe since the Balkan wars.