The United States strongly rejected the vote on Sunday, saying it violated Ukraine's constitution and was carried out "under duress of Russian military intervention".

New York's main contract, West Texas Intermediate for April delivery, was up 23 cents to $99.12 in mid-morning Asian trade, and Brent North Sea crude for May gained 11 cents to $108.32.

Tan Chee Tat, investment analyst at Phillip Futures in Singapore, said US-led Western powers are likely to impose additional sanctions on Russia, which is seen as having orchestrated Crimea's breakaway after a pro-Western government took over in Kiev.

A White House statement said the referendum "would never be recognised by the United States and the international community".

President Barack Obama warned that the United States and its European partners "are prepared to impose additional costs on Russia for its actions".

The Crimean referendum would mark the most radical redrawing of the map of Europe since Kosovo's 2008 declaration of independence from Serbia.

The vote showed 95.5 percent of ballots in favour of the Crimean Peninsula leaving Ukraine to rejoin Russia.

Investors are worried that potential sanctions against Russia could disrupt gas and oil supplies from the country.

More than 70 percent of Russia's gas and oil exports to Europe pass through Ukraine.

Traders are also keeping an eye on the new Federal Reserve's next policy meeting on Tuesday and Wednesday.

The US central bank is expected to further cut its economic stimulus programme as the economy shows further signs of recovery.

The health of the US economy is a key influence on oil prices as the United States is the world's biggest oil consuming nation.