DONETSK, Ukraine — A giant explosion shattered the stillness of an early fall morning here just hours after Ukraine and pro-Russian rebels signed the latest version of their cease-fire accord, illustrating the fragility of a truce agreed upon two weeks ago.

Although shelling and other violence continues daily, both sides maintain that the agreement is still in force. The amendment signed overnight Friday into Saturday was meant to strengthen the truce by requiring both sides to move their artillery nine miles back from the front lines, forming a buffer zone clear of heavy weapons.

Although the artillery fire on Saturday violated the original cease-fire, it was not a direct breach of the new provision because of a 24-hour grace period for its implementation.

The artillery struck a dynamite factory that had served this region’s mining industry, with predictably dramatic results. Two powerful explosions lifted a towering cloud of white smoke over the site that was visible from miles away.