Ukrainian President Petro Poroshenko has vowed to renew operations against pro-Russian rebels, hours after a ceasefire with the separatists in the east of the country expired.
"We will attack and free our lands. The decision not to continue the ceasefire is our answer to terrorists, militants and marauders," he said on his website early on Tuesday.
The fragile ceasefire expired on Monday night. The idea was to give rebels a chance to disarm and to start a broader peace process including an amnesty and new elections.
Kiev had accused the rebels of numerous violations of the ceasefire, and a statement tweeted by the Foreign Ministry said 27 Ukrainian servicemen had been killed since the ceasefire began on June 20.
"The unique chance to put the peace plan into practice was not realised," Poroshenko said in a speech prepared for delivery to the nation, the Associated Press reported.
"This happened because of the criminal actions of the fighters."
The recently elected Poroshenko had already extended the ceasefire from seven days as part of a plan to end the fighting that has killed more than 400 people since April.
Poroshenko's decision to end the ceasefire followed four-way talks in search of a solution with Russian President Vladimir Putin, German Chancellor Angela Merkel and French President Francois Hollande on Monday.
He issued a statement after the talks ended, saying the key conditions needed to continue the ceasefire had not been met.
Poroshenko said he made the decision after a meeting of the national security council.
"After discussion of the situation, I, as commander in chief, took the decision not to continue the unilateral ceasefire."
European leaders and the US have urged Russia to use its influence with the rebels to ease the bloodshed and have threatened to impose another round of economic sanctions against Moscow.
While Putin has expressed support for the ceasefire, the West has accused Russia of allowing weapons and fighters to flow across the border into Ukraine. Russia says any Russians there have gone as private citizens.
The end of the ceasefire raises the question of what action the Ukrainian military can take.
It has so far been unable to dislodge rebels occupying the city of Slovyansk or to retake control of three key border crossings with Russia.
At one point, the rebels shot down a government military transport, killing 49 service members.