Ukraine's armed forces launched a military offensive against the eastern separatists to retake control of an airport in Donetsk, hours after Kiev declared that pro-west businessman Petro Poroshenko had won the country's presidential election.
Following the disrupted elections in the eastern regions, the separatists took over the Sergei Prokofiev International Airport, moving aggressively against Kiev even as Russian forces backed off from the borders, suggesting a Crimea-style annexation was unlikely.
The airport seizure reportedly breached a threshold and Kiev swiftly launched a sharp military operation with air strikes and paratrooper assaults to drive the rebels out.
By late Monday night the military said they had taken control of the airport and cordoned off the premises with roadblocks.
Local media reports said at least 35 people were killed in the operation, but neither the authorities nor the separatists have yet confirmed the casualties.
Monday's offensive was by far the most aggressive military move by Kiev against the rebels thus far.
Poroshenko, the newly elected president, put his weight behind the move, saying military offensives against the "terrorists" should be swift and more effective.
"The anti-terrorist operation should not last two or three months. It should last for a matter of hours," he said at a press conference in Kiev, even as the counter-attack on the Russia-backed separatists was going on.
In a sign that Kiev is toughening its stance against the eastern rebels, the president ruled out dialogue with them, saying no one holds talks with terrorists.
"They want to preserve a bandit state which is held in place by force of arms ... These are simply bandits. Nobody in any civilized state will hold negotiations with terrorists."
The pro-Russian separatists, who had earlier declared the city of one million people an independent republic and seized government buildings, swooped down on the airport in the early hours of Monday.
The armed gang seized the airport and gave an ultimatum to security personnel to leave the premises. However, early in the afternoon Ukrainian forces unleashed an attack on the rebels with fighter jets and helicopter gunfire.
The swiftness and scale of the Ukrainian offensive shook the separatists, who said they would appeal to Russia for help.
"I am forced to appeal directly to Vladimir Vladimirovich Putin for any possible aid ...," Denis Pushilin, speaker of the self-proclaimed Donetsk People's Republic, said, according to the New York Times.
Pushilin claimed separatist fighters were waging pitched battles with the Kiev forces.
"It's a full-blown military standoff. I have no information on casualties. Our groups have destroyed one helicopter of the enemy," he told Reuters.
Moscow, which had apparently softened its stance on the separatist issue in the run-up to the Ukraine presidential elections, said Kiev was engaged in a battle with its own people.
Russia's foreign ministry called on Kiev to halt the military operations, adding that the Organisation for Security and Cooperation in Europe should investigate the military's clashes with the separatists in Donetsk.