Oil prices tumbled on Monday on profit-taking as the market eyed signs of rising U.S. production, though futures remained close to recent mid-2015 highs thanks to last week's decision by OPEC and other producers to extend output cuts.
February Brent crude futures were down $1.30, or 2 percent, at $62.43 a barrel by 2:29 p.m. ET (1929 GMT), while U.S. West Texas Intermediate was down 88 cents, or 1.5 percent, at $57.48.
Brent hit a two-year high of $64.65 a month ago and has since attracted record investment by fund managers.
"Managed money is very long — both futures and options," said Tony Headrick, energy market analyst at CHS Hedging. "If the bulls are not fed, we're subject to a bit of profit-taking that I think we're seeing today."
John Kilduff, a partner at Again Capital Management in New York, said the market could correct slightly, pulling further downward.
"We're in a situation where there might not be much more ammunition on the bullish side," he said.
The market is continuing to watch U.S. crude production, which is nearing a record high, according to data last week.
U.S. output rose in September to 9.5 million barrels per day (bpd), the highest monthly output since 2015, government data shows. On an annual basis, U.S. output peaked at 9.6 million bpd in 1970.
Additionally, drillers in the United States added two oil rigs in the week to Dec. 1, bringing the total count to 749, the highest since September, energy services company Baker Hughes said on Friday.
However, the rig count has been stuck in a narrow range since June. American shale drilling pioneer Harold Hamm on Friday told CNBC he sees the plateauing rig count as a sign that, under pressure from shareholders, oil and gas companies are exercising financial discipline.
That means drillers will be more cautions about spending to increase their production, the Continental Resources CEO told CNBC's "Squawk on the Street."
"That's not the deal anymore, and finally the analysts and everybody else caught on. Shareholders caught on and said, 'We're not going to put money in there just for growth's sake. We want a return, a good return on capital employed,'" he said.
U.S. producers were encouraged during 2017 to increase activity as crude prices started recovering from a multi-year price slump after the Organization of the Petroleum Exporting Countries (OPEC) and some non-OPEC producers, including Russia, agreed to production cuts a year ago.
Last week the producers agreed to extend those cuts of 1.8 million barrels per day (bpd) until the end of next year.
"Market reaction has been positive so far. There are only two worrying aspects ... one is that Iraq's indiscipline has not been discussed, at least not publicly," PVM Oil Associates strategist Tamas Varga said, referring to Iraq's poor compliance with the deal.
"The second is OPEC's own forecast for next year. They are by far the most bullish on 2018, with the annual call on their oil at 33.42 million bpd. This compares with the EIA estimate of 32.70 million bpd and IEA prediction of 32.38 million bpd."
A Reuters survey of output from the 13 OPEC members indicated production fell by 300,000 bpd in November. Supply from the 11 members with production targets under the original accord fell by 230,000 bpd.
The latest agreement allows for producers to exit the deal early if the market overheats. Russian officials had expressed concern that extending the cuts might encourage U.S. shale oil companies, which have been a thorn in OPEC's side, to pump more crude.
— CNBC's Tom DiChristopher contributed to this report.