New York's main contract West Texas Intermediate (WTI) for May delivery rose 46 cents to USD 100.90 a barrel in mid-morning trade and Brent North Sea crude for May gained 31 cents to USD 106.13.
The pro-Kremlin militants on declared independence and vowed to hold a vote on joining Russia, fuelling concerns that Ukraine will fragment after Crimea was absorbed into its giant neighbour last month following a controversial referendum.
Since Russia took control of Crimea, several mainly Russian-speaking eastern regions in the ex-Soviet state have seen calls for similar votes by pro-Kremlin groups.
"The returning tensions in Ukraine are definitely providing some support to oil prices," Desmond Chua, market analyst at CMC Markets in Singapore, told reporters.
"Investors are watching closely for clues about Russia's intentions regarding Ukraine," he added.
Russian troops remain massed on its border with Ukraine, exacerbating fears of a military conflict.
With Ukraine a key conduit for Russian gas to Europe traders fear that any disruption to supplies will send oil and gas prices skyrocketing.
Russian deliveries account for 34 per cent of the natural gas supplies to the European Union, according to the Soufan Group, a US-based intelligence firm.
Singapore-based Phillip Futures said concerns over Ukraine ensured crude prices "stayed well supported at elevated levels despite fears over the influx of Libyan crude into global markets".


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