Ukraine's president has decided to resume a military offensive against pro-Russian separatists in the east of the country.
Western-supported Petro Poroshenko said in an address to the nation: "After examining the situation I have decided, as commander-in-chief of the armed forces, not to extend the unilateral ceasefire. We will attack."
The announcement is likely to trigger a steep escalation in the months-long conflict. It came after a diplomatic push led by France and Germany failed to convince Kiev to extend a 10-day truce that did not quell the fighting in the rebellious east.
Poroshenko is under pressure from the Ukrainian public to toughen his stance on the uprising. But he insisted that Kiev was not abandoning its efforts for peace altogether.
"We are even ready to return to a ceasefire at any moment, when we see that all the parties agree to enact the essential points of the peace plan," he said.
Ukraine and its Western allies accuse Moscow of fuelling the bloodshed that has left about 450 dead by sending arms and fighters across the border between the two ex-Soviet neighbours.
Poroshenko's announcement came a few hours after a conference call with the leaders of Russia, France and Germany, who were pushing for the ceasefire to be extended.
The French presidency had said Kiev and Moscow were working on the "adoption of an agreement on a bilateral ceasefire", raising expectations that the truce would continue.
But Kiev said only that all sides agreed that a new bilateral ceasefire should be discussed at a fresh round of "consultations" involving an envoy from the Organisation for Security and Co-operation in Europe, a Russian diplomat and former Ukrainian leader Leonid Kuchma.
For its part, the Kremlin backed new indirect talks and said President Vladimir Putin had "stressed the importance of extending a ceasefire", to be monitored by international observers.
Before Monday's teleconference, French President Francois Hollande and German Chancellor Angela Merkel reiterated a European Union warning of more sanctions against Moscow unless the Kremlin explicitly pressured pro-Kremlin rebels to stop fighting.
Broader sanctions could cut off whole sectors of the Russian economy from the EU's 500 million consumers, pushing it into recession. The International Monetary Fund is already warning of negligible growth.
There was no immediate reaction from the separatists, but the Interfax-Ukraine news agency cited unnamed insurgent commanders as saying that shelling had started in the rebel-held town of Kramatorsk.
In the talks between the four leaders, some headway appeared to be made on the contentious issue of shoring up Ukraine's border with Russia.
Putin was "ready to authorise Ukrainian border guards access to Russian territory" to help monitor the frontier, Steffen Seibert, a spokesman for Merkel said.
On Monday, Russian state television station Channel One said that its cameraman Anatoly Klyan, 68, died after being shot in the stomach by Ukrainian troops while on an overnight reporting trip with insurgents at a military base near the rebel-controlled city of Donetsk.
Meanwhile, Ukrainian authorities claimed 27 servicemen were killed during the ceasefire and almost 70 injured.
Poroshenko has been coming under growing public pressure to launch a full-scale assault, with 500 people protesting against the ceasefire outside his office on Sunday.