Chocolate baron Petro Poroshenko claimed a resounding victory on Sunday in Ukraine's presidential election and immediately vowed to end a bloody pro-Russian uprising that thwarted voting across swathes of the separatist east.

The pro-Western self-made billionaire won close to 56 percent of a vote held while Ukraine is threatened by disintegration and financial collapse.

"My first decisive step will be aimed at ending the war, ending chaos, and bringing peace to a united and free Ukraine," the 48-year-old said at a press conference in Kiev.

"I am certain that our decisive actions will bring fairly quick results," he said in reference to an outcome that analysts said hands him a strong mandate to deal directly with Russian President Vladimir Putin.

Despite his commanding lead, the armed rebellion meant voting was largely blocked in two eastern regions that make up 15 percent of the national electorate -- raising concerns about the legitimacy of his mandate across the entire country.

Official confirmation of the results should avoid the need for a June 15 runoff that would have extended political uncertainty at the most painful moment in Ukraine's 23-year post-Soviet history.

The exit polls put the political veteran more than 30 points ahead of nearest rival Yulia Tymoshenko, the former prime minister who spearheaded the 2004 pro-democracy Orange Revolution but then became embroiled in corruption scandals that saw her put behind bars by the old pro-Russian regime.

'Courageous Ukrainians'

US President Barack Obama praised Ukrainians for showing courage by voting in the face of the threat posed by militants who have seized about a dozen cities and towns in a seven-week rebellion.

"Despite provocations and violence, millions of Ukrainians went to the polls throughout the country, and even in parts of eastern Ukraine, where Russian-backed separatist groups sought to disenfranchise entire regions, some courageous Ukrainians still were able to cast their ballots," the White House quoted Obama as saying.

Putin issued no immediate comment and instead spent Sunday evening in Belarus watching his country compete in the final of the ice hockey world championship.

He had rejected the authority of Kiev's Western-backed interim leaders and had only promised to "respect" the outcome of Sunday's vote.

Poroshenko vowed to pay his first trip to the east where insurgents have proclaimed their own independent republics in a rebellion that has already cost scores of lives.

Ukrainians voted en masse in the capital Kiev and the west but were kept from doing so by armed militants who blocked polling stations across most of the east.

"We consider Poroshenko -- if he is elected -- to not be legitimate," said former pro-Russian presidential candidate Oleh Tsarov.

"We consider that the winner of the election is president of west Ukraine -- he is a half president."

'Long list of problems'

Poroshenko said Ukraine had a "long list of problems" to discuss with Russia through negotiations that should also involve the EU and Washington.

Ukraine is fighting for its very survival after Putin responded to the popular overthrow of a Kremlin-backed leader by seizing Crimea and threatening to invade the rest of Ukraine to "protect" the country's ethnic Russian community.

As well as trying to reunite the country, the new leader will have to set into motion overdue economic restructuring measures that world lenders are demanding in return for $27 billion (20 billion euros) in aid to stave off bankruptcy.

But UniCredit's chief economist Erik Nielsen said the election gave Ukraine only "a glimmer of hope" at avoiding a still deeper crisis.

The insurgency in the Russian-speaking east -- home to seven million of Ukraine's 46 million people and most of its heavy industry -- has unleashed the worst chill in East-West relations since the Cold War.

Turnout was strong in Ukrainian speaking areas but was down to a trickle in eastern cities such as Donetsk where masked gunmen in green fatigues patrolled the streets.

But voters also dealt a stinging rebuke to Ukraine's ultranationalist forces who had been blamed for most of the violence against ethnic Russians in the east.

Exit polls showed both Pravy Sektor (Right Sector) paramilitary group leader Dmytro Yarosh -- branded a "fascist" by Moscow -- and nationalist Svoboda (Freedom) party leader Oleh Tyahnykob drawing less than two percent of the vote.

The ballot was called after Kremlin-allied president Viktor 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, threatened with further US and EU sanctions, appeared to make a major concession Friday by saying he was ready to work with the new Kiev team.

"We understand that the people of Ukraine want their country to emerge from this crisis. We will treat their choice with respect."

Russia also said it had started withdrawing from Ukraine's border around 40,000 soldiers whose presence had raised deep Western suspicions and prompted NATO to send additional fighters to former Soviet satellite nations such as Poland and the Baltic states.

However on polling day the Kremlin infuriated Kiev when it dispatched Prime Minister Dmitry Medvedev to Crimea for a surprise visit.

On the ground, one person was reported killed near a polling station in the east but no major fighting was reported.

Violence had flared Saturday in the flashpoint of Slavyansk, a rebel stronghold where an Italian photographer and his Russian translator were killed after being caught in a gun battle.