SNIZHNE, Ukraine — An airstrike in eastern Ukraine sent an apartment building crumpling to the ground Tuesday, killing at least 11 people and adding to the steadily mounting civilian death toll from the fighting between government forces and pro-Russian insurgents.

Rebels pinned the attack on the Ukrainian air force. The government swiftly denied blame but was not immediately able to offer an alternative explanation.

The bombing in the rebel-held town of Snizhne demonstrated how airstrikes and heavy rocket fire are becoming increasingly common as the conflict drags into its fourth month. The attack comes one day after a Ukrainian military transport plane was shot down in disputed circumstances.

The devastation in Snizhne bore signs of a strike by several missiles and left only a mountain of smoking debris. The four-story apartment block appeared to have been hit in two separate spots, causing the collapse of several tiers. A nearby house was also destroyed.

Resident Igor Chernetsov lost his wife in the attack. “I heard an explosion, and suddenly I was thrown out of the apartment, out of the fourth floor,” said Chernetsov, his head swathed in a bandage. “I woke up covered in dust and had no idea what had happened.”