Brent crude tumbled more than 3 percent on Monday on speculation of a nuclear deal that could boost Iran's oil exports.
U.S. crude fell 17 cents, or 0.3 percent, to settle at $49.59 a barrel. It rose about 1 percent earlier as traders bet the gap between the two would narrow.
Brent crude was down 2.9 percent, at $59.65. Front-month Brent jumped 18 percent in February, the largest monthly rise since May 2009.
Iranian Foreign Minister Mohammad Javad Zarif said a deal on Iran's nuclear program could be concluded this week if the United States and other Western countries had sufficient political will and agreed to remove sanctions on Tehran.
Iranian oil exports have been restricted by sanctions for several years as the United States and Europe responded to Tehran's nuclear program, although Iran says its nuclear plans are peaceful.
Analysts say Iran could increase its oil sales fairly quickly if sanctions were lifted and may eventually be able to raise exports by up to 1 million barrels per day. A Reuters survey last week showed Iran pumped around 2.8 million bpd in February.
Brent was also pressured initially by rising Libyan output and a firmer dollar that weighed on greenback-denominated commodities.