The premium of Brent versus U.S. crude narrowed to $12.32 earlier in the Asian trading session, the narrowest since July, but later widened to around $13 per barrel, little change on the day.
Analysts said concerns about crude supplies to U.S. Gulf Coast refineries were affecting WTI prices on Monday after Exxon Mobil was forced to close a pipeline due to a leak.
Exxon's Pegasus pipeline, which can carry more than 90,000 barrels per day (bpd) of crude from Illinois to Texas.
"This leak prompted a very fast reaction and reinforced the concerns of opponents of the Keystone XL pipeline, which is currently awaiting U.S. government approval," David Wech from JBC Energy consultancy said.
The proposed 800,000 bpd Keystone XL pipeline would carry heavy crude from Canada's tar sands to the Gulf Coast refining hub. Environmentalists have expressed concerns about the impact of developing the oil sands and say the crude is more corrosive to pipelines than conventional oil.
Wech said the spread between Brent and WTI could narrow further after South Korea confirmed it would close a tax loophole, which had encouraged imports of North Sea crude.
"We could see that support fade given the tax change," he said. (Editing by Clarence Fernandez and Jane Baird)