A fierce battle erupted Monday for control of the main airport in rebel-held eastern Ukraine in a dramatic escalation of the conflict just hours after president-elect Petro Poroshenko vowed he would not let the country become another Somalia.

As darkness fell in Donetsk, a city of about 1 million, it was unclear who was in control of the airport. Hundreds of fighters of the separatist Donetsk People’s Republic had been brought by trucks to a wooded area on the fringes of the airport, many of them armed with rocket-propelled grenade launchers and automatic rifles. At least one warplane streaked over the city, firing flares, and explosions were heard from the direction of the airport.

Ukrainian fighter jets and combat helicopters struck the terminal building at the airport to try to dislodge scores of separatist gunmen who seized the complex, triggering hours of heavy firefights.

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.

Russia has heavily criticized an offensive by Ukraine’s military against the rebels, and Poroshenko indicated he wants it to end quickly.

“The anti-terrorist operation cannot and should not last two or three months,” he said. “It should and will last hours.”

The fighting flared on very day that Poroshenko — who has pledged to try to bring peace and unity after months of turmoil — was formally declared the winner of Sunday’s presidential election with 54 percent of the vote.

The Donetsk city mayor said there had been “civilian casualties” in the battles and told local residents to stay indoors.

Plumes of thick black smoke rose from the airport complex as the sound of explosions and heavy machinegun fire rang out throughout the day.

There was no confirmation of casualties but AFP correspondents saw the body of a woman near the Donetsk train station, although it was not clear how she died.

Scores of gunmen had stormed the airport in an apparent show of defiance against Poroshenko, underscoring the daunting challenges ahead to prevent the former Soviet state from plunging into all-out civil war.

Ukraine’s election chief confirmed the billionaire chocolate baron president, saying his 41-point lead over closest rival, former prime minister Yulia Tymoshenko, was unassailable with only a tenth of the ballots left to count.

Even before the results, the 48-year-old Poroshenko had moved swiftly to stamp his authority as Ukraine’s new leader, vowing to rebuild relations with Russia while steering the country on a westward path.

He said Ukraine would press on with its offensive against insurgents who now control about a dozen cities and towns in the east, and insisted there would be “no talks with terrorists”.

“Their goal is to turn Donbass (east Ukraine) into Somalia.”

Sunday’s vote was seen as the most important since Ukraine’s independence in 1991 as it fights to stay united after months of turmoil and avert economic collapse after years of Soviet-era mismanagement and rampant corruption.

Poroshenko was seen as the consensus candidate, although he has switched sides during his political career — serving as a minister in the government of pro-Kremlin president Viktor Yanukovych before becoming a major funder of the bloody uprising that ousted him.

But the insurgency, which has already cost at least 150 lives, thwarted polling in much of the east and rebels have defiantly refused to recognise the result.

Russia, threatened with more Western sanctions if it meddled further in Ukraine after its seizure of Crimea in March, said it was willing to work with the new leader.

“We are ready for pragmatic dialogue, on an equal footing, based on respect for all agreements, in particular in the commercial, economic and gas spheres,” Foreign Minister Sergei Lavrov said. “We respect the result of the choice of the Ukrainian people.”

Rebels have rejected dialogue with Poroshenko, with Donetsk separatist leader Denis Pushilin saying talks were only possible if Russia mediated and calling for a prisoner swap and a withdrawal of Kiev’s troops.

While turnout was strong across the capital Kiev and the more pro-European west on Sunday, voting was largely blocked in Donetsk and Lugansk, two regions that make up 15 percent of the electorate.

However observers with the Organisation for Cooperation and Security in Europe said the election “largely upheld democratic commitments” and provided the new leader with legitimacy.

US President Barack Obama had praised “courageous Ukrainians” for voting in the face of the militant threat and said Washington looked forward to working with the new president.

The ballot was called after Yanukovych — his corruption-stained regime long a source of discontent — was ousted in February in the bloody climax of months of protests sparked by his rejection of a historic EU pact.

Putin responded by seizing Crimea and threatening to invade the rest of Ukraine to “protect” the country’s ethnic Russian community, raising the prospect of all out war on Europe’s doorstep.

But Russia said last week it had started withdrawing from Ukraine’s border around 40,000 soldiers whose presence had raised deep Western suspicions.

In addition to the political and security headaches ahead, Poroshenko will have to adopt painful austerity measures being demanding in return for $27 billion (20 billion euros) in international aid to prevent bankruptcy.

But in a sign of one positive development, the EU energy commissioner said Russia and Ukraine had a “good chance” of striking a deal to prevent Moscow cutting off supplies as threatened next month.