Brent crude futures fell 20 cents, or 0.3 percent, to $74.54 a barrel by 11:48 a.m. EDT (1548 GMT). This month, the global benchmark hit highs above $75, a level last seen in late 2014.
US West Texas Intermediate (WTI) crude futures fell 30 cents to $67.89 a barrel, a 0.4 percent loss.
Brent was on track for a weekly gain of about 0.7 percent, while WTI was set for a weekly loss of about 0.7 percent.
US President Donald Trump will decide by May 12 whether to reimpose sanctions on Iran that were lifted as part of an agreement with six other world powers over Tehran's nuclear program. The renewed sanctions would likely dampen Iranian oil exports, disrupting global oil supply.
"That's an issue that is more political in nature that could have a shock in the market," said Mark Watkins, a regional investment manager at US Bank Wealth Management in Park City, Utah." "It's one of those wildcards that's out there because if the sanctions do happen, there's going to be oil that comes off the market."
Brent has risen by around 5 percent this month. The gains came despite a higher dollar, which is at its strongest since Jan. 11 against a basket of currencies.
A stronger dollar makes greenback-denominated commodities more expensive for holders of other currencies.
Concerns about market tightness have also been fueled by the deteriorating political and economic situation in Venezuela that has led to a 40 percent decline in crude output in the past two years.
Price increases have been capped by rising US production as shale drillers ramp up activity, underpinning a widening discount between Brent and WTI. US crude's discount to Brent hit its widest since Dec. 28 at $6.74 a barrel.
Surging US production, which rose to 10.59 million barrels per day last week, has encouraged record-high US exports.
Market analysts were awaiting US rig count data from General Electric Co's Baker Hughes energy services firm, due to be released later on Friday.
Weak refining margins hurt two of the world's largest integrated energy companies for the second consecutive quarter, although Chevron Corp's oil production gains in the first quarter outshone its larger rival Exxon Mobil Corp.