Benchmark West Texas Intermediate crude for June delivery fell 27 cents to close at US$99.99 on the New York Mercantile Exchange. The contract ended last week at $99.76.
Brent crude, a benchmark for international varieties used by many U.S. refineries, declined 15 cents to close at US$107.89 in London.
Crude supplies in the U.S. fell this week, which surprised traders and gave some support to prices. But supplies remain ample and refineries are pumping out enough fuel to meet demand. That suggests refineries won't need to quickly draw down supplies in the coming weeks.
Overseas demand for crude appears muted and supplies are plentiful, but worries over the political turmoil in Ukraine and continued disruption of oil exports from Libya have kept Brent prices up.
"Libya's two most important oil terminals, Ras Lanuf and Es Sider, will remain shut for the foreseeable future, which is likely to continue to severely hamper the supply of oil," said analysts at Commerzbank in Frankfurt in a note to clients.
Meanwhile, pro-Russian insurgents in eastern Ukraine are planning a referendum on independence over the weekend, in apparent defiance of a call by Russian President Vladimir Putin to put off the vote.
Traders worry Russian energy exports could be interrupted if further instability in Ukraine results in stronger western sanctions against Russia.
In other energy futures trading on the Nymex, wholesale gasoline fell 0.9 of a cent to close at US$2.896 a U.S. gallon (3.79 litres), heating oil fell 1.3 cents to close at US$2.907 a gallon and natural gas fell 4.1 cents to close at US$4.531 per 1,000 cubic feet.
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