Petro Poroshenko ended a 10-day ceasefire in the conflict with pro-Russian separatists in the east of the country, saying he would send military forces back on the offensive after talks with Russia and European leaders failed to start a broader peace process.
Poroshenko's decision, announced shortly after the much-breached ceasefire expired, raises the prospect of renewed escalation of a conflict that has already killed more than 400 people.
A grave Poroshenko made a televised address vowing that "We will attack and liberate our land. Not renewing the ceasefire is our response to terrorists, rebels, looters."
The ceasefire expired at 10 pm on Monday.
There was no immediate sign of a response from Russia early on Tuesday.
The idea behind the truce – which was announced on 20 June – was to give pro-Russian rebels a chance to disarm and to start a broader peace process including an amnesty and new elections.
But the ceasefire has been repeatedly breached since Poroshenko declared it, with both sides blaming the other for breaking the truce.
Meetings of a contact group including representatives of Kiev, Moscow and the self-declared Donetsk and Luhansk people's republics won official recognition of the ceasefire even as fighting on the ground continued.
In his remarks, the president accused the rebels of breaking the ceasefire more than 100 times.
Poroshenko said the government could return to the ceasefire "at any moment, when we see that all sides are keeping to the basic points of the peace plan."
But his speech appeared to mark the start of a new campaign against the rebels and a further escalation of the conflict in the east.
"Peace is, was and will be my goal," he added.
"Only the instruments of achieving it are changing ... The defence of Ukraine's territorial integrity, of the security and lives of peaceful citizens, demands not just defensive but offensive action against the terrorist militants."
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. Ending the ceasefire, this is our answer to terrorists, armed insurgents and looters, to all who mock the peaceful population, who are paralysing the economy of the region ... who are depriving people of a normal, peaceful life."
Poroshenko's decision followed four-way talks in search of a solution with Russia's president, Vladimir Putin, the German chancellor, Angela Merkel, and the French president, Francois Hollande, on Monday as the deadline approached.
He issued a statement after the talks ended, saying the key conditions needed to continue the ceasefire had not been met.
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 sending weapons and fighters across the border into Ukraine.
Russia says any Russians in Ukraine have gone there as private citizens.
Tension between Russia and Ukraine escalated in February when protests by people who wanted closer ties with the European Union drove pro-Russian president Viktor Yanukovych from office.
Russia called that an illegal coup and seized Ukraine's Crimea region, saying it was protecting the rights of people there who speak Russian as their main language.
The insurrection in the eastern regions near the Russian border started soon afterward, with separatists occupying buildings and declaring independence.
The end of the ceasefire raises the question of what action the Ukrainian military can take.
It has been unable to dislodge rebels occupying the city of Slavyansk 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.
Source: The Guardian Today