* Washington adds 20 prominent Russian business men to sanctions list * Oil trader Gunvor co-founder Timchenko included in list * Gains capped by stronger U.S. dollar, seasonal slump in demand (Updates throughout, previous dateline SINGAPORE) By David Sheppard LONDON, March 21 (Reuters) - Brent crude rose towards $107 per barrel on Friday as fresh U.S. sanctions against Russia, the world's second-largest oil exporter, increased fears of a disruption to supplies. Brent remained on track for a fourth weekly loss, however, with a seasonal slump in demand leading to a near 5 percent slide since the beginning of March, when prices briefly jumped to a three-month high above $112 as Russia took control of Ukraine's Crimea region. Washington expanded sanctions to 20 more prominent Russians and allies of Russian President Vladimir Putin on Thursday, including Gennady Timchenko, co-founder of oil trading firm Gunvor. Within hours Gunvor announced Timchenko had sold his near 50 percent stake in the company to allow the firm, which ships almost 3 percent of global oil supplies, to avoid disruptions to its operations. "The uncertainty is definitely back," Abhishek Deshpande, commodities analyst at Natixis in London said. "The U.S. has shown a willingness to intensify sanctions against Putin's inner circle, and may impose targeted penalties on businesses, including the energy sector." Brent for May delivery was up 34 cents at $106.79 per barrel by 0923 GMT, after settling 60 cents higher on Thursday. U.S. crude for May delivery, which became the front-month contract on Friday, was 12 cents higher at $99.02 per barrel. Gains were capped as the U.S. dollar rose near a three-week high against a basket of major currencies. A strong dollar makes commodities priced in the greenback more expensive for holders of other currencies. GUNVOR While Timchenko said he had sold his stake in Gunvor before being sanctioned by the United States on Thursday, the fact that a major energy trader has been dragged into a growing political stand-off over Ukraine is adding to market concerns. Gunvor, which had a turnover of $93 billion in 2012, grew rapidly by trading large volumes of oil from Russian state companies such as Rosneft at the end of last decade, but has since then ceded its leading positions and now focuses on trading in Europe and Asia. "Anyone in the business looking out at the horizon suddenly see clouds that look very serious, even if they aren't anywhere close," said Kevin Book, energy policy analyst at ClearView Energy Partners in Washington. President Barack Obama threatened broad penalties against sectors of Russia's economy if Moscow moves deeper into Ukraine. Senior administration officials said many parts of the Russian economy could be targeted, including energy, defence, mining and financial services sectors. European leaders on Thursday added 12 people to a list of those subject to travel bans and asset freezes for their part in Russia's seizure of Crimea and will begin preparations for trade and economic measures if Russia expands its footprint in Ukraine. (Additional reporting by Jacob Gronholt-Pedersen; editing by Jason Neely)

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