ARTEMIVSK, Ukraine — As fighting erupted throughout eastern Ukraine on Friday before a cease-fire begins on Saturday night, the United States accused Russia of joining separatist rebels in an all-out attack on Ukrainian forces around the contested town of Debaltseve.

When the pact was signed with a two-day window before the truce, some last-minute jockeying for position was expected. But the intensity and scope of the violence raised concerns that the agreement signed this week, rife with ill-defined and ambiguous provisions, might prove as ineffective as the first cease-fire pact, signed in September.

Artillery shelling and gunfire reverberated in the area around Debaltseve, a strategic rail hub where rebels were said to have severed the last land route into town, leaving government forces surrounded. At least 18 people were reported to have died there.

Fighting flared in many towns. Two people were killed and six wounded in an artillery strike in the Ukrainian-controlled town of Shchastya, near rebel-held Luhansk.