Kyiv says Russian-backed separatists have "cynically" violated a cease-fire in eastern Ukraine by firing an antitank missile.
The Ukrainian military said the October 7 attack wounded four soldiers.
The attack undermines earlier comments by President Petro Poroshenko, who said a "real truce" had begun in the country's east, but warned that peace was still a long way off.
Addressing students undergoing military training in Kyiv, Poroshenko said the end of war will come "when the last piece of the Ukrainian land is liberated from the enemy, the occupant, the invader," he said.
A cease-fire agreed in Minsk in February was often broken, but has been widely upheld in the past month.
The conflict between Ukrainian government forces and rebels has killed more than 7,900 people since April 2014.
Poroshenko's remarks come a day after the rebels delayed until next year local elections they had planned for the coming weeks in territory they control in the Donetsk and Luhansk regions.
The European Union called the move a "fundamental step" toward full implementation of the Minsk accord aimed to resolve the conflict.