Within hours of Mr Poroshenko's announcement yesterday morning, the military went into action against rebel bases and checkpoints in the east which has been in separatist ferment since April.
Saying Ukrainian forces had launched attacks "from the air and land", the defence ministry said: "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."
Mr 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 four-way telephone talks involving the German and French leaders and Russia's Vladimir Putin.
Showing impatience at what he had heard from Mr Putin, Mr Poroshenko said Ukraine had not seen "concrete steps for de-escalating the situation, including strengthening controls on the border".
In Moscow, the foreign ministry hinted the US was behind Mr Poroshenko's decision not to extend the ceasefire.
"There is an impression 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.
The speaker of Russia's lower house of parliament, Putin ally Sergei Naryshkin, called for a new ceasefire, saying: "We think that without a truce, without the start of dialogue, it is simply impossible to restore peace, justice and law and order in Ukraine."
Mr Poroshenko, just over three weeks in office, is facing rising anger at home and from the new political establishment over military losses and is under pressure to switch to more forceful action against the rebels.