Ukrainian forces struck at pro-Russian separatist bases in eastern regions with air and artillery strikes on Tuesday after President Petro Poroshenko announced he would not renew a ceasefire but go on the offensive to rid Ukraine of "parasites".
His decision quickly drew fire from Russian President Vladimir Putin who said Poroshenko had disregarded the advice of himself and German and French leaders. Putin said Poroshenko would now have to bear full responsibility for veering off the road to peace.
Repeating a threat he made in March when Russia annexed Crimea, Putin said Moscow would continue to defend the interests of ethnic Russians abroad - up to three million of whom live in the east of Ukraine, which has been in separatist ferment since April.
The United States said the separatists had not abided by the ceasefire and Poroshenko had "a right to defend his country".
Within hours of Poroshenko's early morning announcement, his military went into action against rebel bases and checkpoints, bombarding them from the air and with artillery.
"The terrorists' plan to significantly escalate armed confrontation has been disrupted and the threat of losses to the civilian population and service personnel has been liquidated," the defense ministry said.
There was no immediate word on casualties.
Poroshenko, who accuses Russia of fanning the conflict and allowing fighters and equipment to cross the border to support the rebels, turned his back on another renewal of a 10-day unilateral ceasefire after the phone talks involving Putin, German Chancellor Angela Merkel and French President Francois Hollande.
Showing impatience at what he had heard from Putin, Poroshenko said in his early morning statement that Ukraine had not seen "concrete steps for de-escalating the situation, including strengthening controls on the border".
Poroshenko, just over three weeks in office, faces a possible popular backlash at home over military losses during the ceasefire and was under pressure to switch to more forceful action against the rebels.
'Militants and marauders'
Many of his security advisers told Poroshenko that the rebels had used the June 20 ceasefire, renewed for three days on June 27, to regroup and rearm.
A statement tweeted by the Ukrainian foreign ministry on Monday, as Poroshenko went into talks with his security chiefs, said 27 Ukrainian servicemen had been killed and 69 wounded since the start of the ceasefire.
Announcing the military would now act to answer the "terrorists, militants and marauders", Poroshenko accused the rebels of failing to keep to the truce or follow a peace plan he had outlined.
Later on his Facebook page, the 48-year-old leader warned the future would be difficult, adding: "we must be united, because we are fighting to free our land from dirt and parasites."
Putin bluntly suggested Poroshenko had been isolated in Monday's phone-in with himself, Merkel and Hollande.
"Unfortunately President Poroshenko took the decision to restart military operations and we – I mean myself and my European colleagues – could not convince him that the road to stable, strong and long-lasting peace does not lie through war," he said.
"Up until now (Poroshenko) was not directly linked to the order to start military operations but now he has taken on this responsibility fully, not only militarily but also politically," he said.
It was not immediately known whether Berlin and Paris agreed with this version of Monday's discussions.
U.S. State Department spokeswoman Marie Harf said Washington supported Poroshenko's move.
"It takes two to keep a ceasefire," she told a regular news briefing. "President Poroshenko put in place a seven-day ceasefire; he abided by it, but the fact remains that the separatists, many of them weren't adhering to it, and he has a right to defend his country."
She said U.S. Secretary of State John Kerry had spoken on Monday to Poroshenko, who had said he was still committed to a peace plan.
"So the ultimate goal here is to get back to a ceasefire, to get back to a peace plan, but it takes two parties to put that in place and to keep it in place," Harf said.
Before Putin spoke, the Russian foreign ministry hinted that the United States was behind Poroshenko's decision. "There is an impression that the change in Kiev's position ... could not have come about without influence from abroad, despite the position of leading EU member states," it said in a statement.
Air strikes, artillery
"After the president's (Poroshenko's) speech, the ATO (Anti-Terrorist Operation) went into action. We opened artillery fire, carried out air strikes at the strategic points of the terrorists and places where they are concentrated," Ukrainian military spokesman Oleksiy Dmytrashkovsky said.
Rebels had fired on a SU-25 attack aircraft, damaging it, but the plane had managed to land safely at its air base, he said. He denied a rebel report that a military helicopter had been brought down.
One Ukrainian serviceman had been killed and 17 wounded in the past 24 hours in rebel attacks on Ukrainian posts, Dmytrashkovsky said.
Poroshenko expressed willingness to return to a ceasefire if it became clear all sides were ready to carry out all aspects of the peace plan, including the freeing of hostages and creating effective border controls.
Poroshenko had extended a government ceasefire last week until 10 p.m. (1900 GMT) on Monday to allow for peace talks with a contact group involving separatist leaders, a former Ukrainian president, a senior representative of the OSCE rights and security body and Moscow's ambassador to Kiev.
"The unique chance to implement the peace plan was not realized. It happened because of the criminal actions of the militants. They publicly declared their unwillingness to support the peace plan as a whole and in particular the ceasefire," Poroshenko said.
Pro-Russian separatism erupted in Ukraine's east in April after street protests in Kiev toppled a Moscow-backed president, Viktor Yanukovich, when he had walked away from a free trade deal with the European Union that would shift Ukraine westwards.
Russia subsequently annexed Crimea and separatist rebels in Ukraine's Russian-speaking east seized buildings and strategic points, declaring "people's republics" and saying they wanted union with Russia.
Poroshenko last Friday signed the EU deal which Yanukovich baulked at in defiance of threats by Russia to carry out retaliatory trade action.
Moscow could face more penalties from the EU on top of existing asset freezes and visa bans unless pro-Russian rebels act to wind down the crisis in the Ukraine's Donetsk and Luhansk provinces.
But EU diplomats said in Brussels that EU governments would not immediately take action over new sanctions on Russia because the situation on the ground was confused following the renewal of fighting.