Already battered and giving ground after the conflict flared up a month ago, and fighting against forces they and Western governments say include the Russians, Ukrainian soldiers had been trying at least to hold onto their supply line until the cease-fire took effect, at midnight Saturday. It seems increasingly evident that they failed, putting the agreement in doubt.

Previously, this section of road was known to be mined and within range of rebel artillery, though not occupied by rebel soldiers. The video, with a backdrop of blown-up tanks and dead Ukrainian soldiers, appeared to show Russian-backed militants on the road near Logvinovo, about five miles from Debaltseve.

Having apparently lost on the battlefield, Ukraine now will appeal for diplomatic pressure on Russia to prevail upon the separatists to open the road. But European leaders and President Vladimir V. Putin of Russia were unable to solve the Debaltseve riddle during intense negotiations in Minsk last week, and it remains an open question whether they can now.