Oil prices rose on Monday after pro-Russian rebels in eastern Ukraineclaimed a majority of voters in a disputed referendum chose independence, raisingfears of a civil war that could disrupt supply.The US benchmark, West Texas Intermediate for delivery in June, added 40 cents to$100.39 a barrel.Brent North Sea crude for June gained 49 cents to trade at $108.38 a barrelapproaching midday in London."With the risk of escalating geopolitical tensions in and around Ukraine ... oilsupplies could be disrupted," said Desmond Chua, analyst at CMC Markets inSingapore.Rebels in the Donetsk province of Ukraine on Sunday said a total of 89 percent of voters there cast ballots in favour of self rule.There was no immediate word of results from Lugansk, the other province holding asimilar referendum.But the vote for independence there was expected to be similar to Donetsk's, or evenexceed it. The two regions are home to seven million people, out of Ukraine's totalpopulation of 46 million. The West fears these disputed votes could hasten the break-up of the former SovietRepublic and lead to a civil war on Europe's eastern edge, disrupting supplies andsending energy prices rocketing.Singapore-based Phillip Futures said investors are also on edge due to Russiandemands last week that Ukraine pay upfront for its future natural gas deliveries,because of billions of dollars in outstanding debts.The announcement imperils supplies to a large swathe of the European Unionbecause nearly 15 percent of all Russian gas consumed by the 28-nation bloc transits Ukraine.The US and its Western European allies supporting Ukraine's Western-friendlygovernment have accused Russia of fomenting unrest in the country's east after thefall of former Pro-Russian president Viktor Yanukovych in February. Moscowstrongly denies the allegations.