Ukrainian fighter jets and combat helicopters struck the terminal building at Donetsk airport to try to dislodge separatist gunmen who seized the complex, triggering heavy gun battles.
It was the most forceful action by the Kiev government in its battle to crush a bloody pro-Moscow insurgency that has raged in the industrial east since early April, threatening to tear apart the former Soviet state.
Plumes of thick black smoke rose from the airport complex as the sound of explosions and heavy machinegun fire rang out throughout much of the day, AFP correspondents at the scene said.
Ukraine's election chief confirmed Poroshenko as the president, saying his 54 per cent of the vote and 41-point lead over the second-place holder were unassailable with only a tenth of the ballots left to count.
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"We can now already draw a fairly important conclusion: there will be no need for a second round. On May 25, 2014, a new president was elected in Ukraine," election chief Mykhailo Okhendovskyi said.
Poroshenko has moved swiftly to stamp his authority as Ukraine's new leader, and the country's former master Moscow said Monday it was ready to work with him.
"There are no talks with terrorists," said the centrist pro-Western tycoon known as the chocolate king for his confectionary empire.
"Their goal is to turn Donbass (east Ukraine) into Somalia. I will not let anyone do this to our state and I hope that Russia will support my approach."
Yesterday's vote was seen as the most important in Ukraine's post-Soviet history as it fights to stay united after months of turmoil and avert economic collapse.