Light, sweet crude for June delivery was flat at $66.27 a barrel in Asian electronic trading on the New York Mercantile Exchange midmorning in Singapore. It had gained as much as 5 cents in early trade. On Monday, the contract climbed $1.33 to $66.27 per barrel.

Worries that violence could rise in Nigeria ahead of the inauguration of a new president supported prices. Nigeria is one of the world's largest crude producers, and attacks on oil installations have become an almost daily occurrence following elections there last month.

"Profit-taking may trim prices for a while," said Koichi Murakami, an analyst with brokerage Daiichi Shohin. "But later, I think they will be supported by the Nigerian concerns."

Attackers in Nigeria vandalized an oil installation run by Total SA, the company said Monday. No injuries, kidnappings or production cuts were reported.

Traders are also worried about gasoline supplies heading into the peak Northern Hemisphere summer driving season which kicks off this weekend in the U.S.

The U.S. Energy Information Administration reported last week that domestic gasoline inventories while increasing to 195.2 million barrels for the week ended May 12 are still well below the average for this time of year.

The market was also eyeing the U.S. government's release of its updated hurricane outlook this week.