US West Texas Intermediate (WTI) crude futures were at $61.40 a barrel by 1140 GMT. That was 61 cents, or 1 per cent, below their last close. WTI hit $62.21 the previous day, which was its strongest since May 2015.
Brent crude futures lost 50 cents, or 0.7 per cent, to $67.57. The previous day it touched $68.27, also the highest since May 2015.
Traders said political tensions in Iran, the third-largest producer in the Organization of the Petroleum Exporting Countries (OPEC), had pushed prices higher.
"The protests in Iran add more fuel to the already bullish oil market mood," said Norbert Ruecker, head of commodity research at Swiss bank Julius Baer.
Oil prices have received general support from production cuts led by OPEC and Russia, which started in January last year and are set to last through 2018, as well as from strong economic growth and financial markets.
That has helped to tighten markets. US commercial crude inventories fell by 7.4 million barrels in the week to December 29, to 424.46 million barrels, according to data from the Energy Information Administration (EIA).
That is down 20 per cent from peaks last March and close to the five-year average of 420 million barrels.
Yet given Iran's oil production has not been affected by the unrest and that US output is soon likely to break through 10 million barrels per day (bpd), a level so far reached only by Saudi Arabia and Russia, doubts are emerging whether the bull run can last.
Bank Jefferies said the oil price "upside from here is not obvious to us" but added that it expects the oil market to remain undersupplied through 2018.
Julius Baer's Mr Ruecker said that crude prices above $60 project an "overly rosy picture".
"Oil production disruptions (in Iran) remain a very distant threat ... disruptions in the North Sea have been removed ... (and) US oil production surpassed the 2015 highs in October and is set to climb to historic highs this year," he said.
Lukman Otunuga, analyst at futures brokerage FXTM, struck a similarly cautious tone, saying: "While the current momentum suggests that further upside is on the cards, it must be kept in mind that US shale remains a threat to higher oil prices."