Global benchmark Brent crude futures were down 13 cents, or 0.2 per cent, at $ 74.61 a barrel by 0050 GMT, after rising 1 per cent on Thursday
US West Texas Intermediate (WTI) crude fell 12 cents, or 0.2 per cent, to $ 68.07 a barrel. The contract gained 0.2 per cent the previous session.
Brent is heading for a third week of gains, up around 0.7 per cent, while WTI faces a small weekly decline.
“Markets continued to price in potential disruptions to world oil supplies if the United States withdraws from the 2015 Nuclear Accord with Iran,” ANZ Bank said in a note.
US President Donald Trump will decide by May 12 whether to restore sanctions on Iran that were lifted after an agreement over its disputed nuclear programme, which would probably result in a reduction of Iranian oil exports.
Brent has gained more than 6 per cent this month on expectations the United States will renew sanctions.
Concerns about market tightness have also been fueled by the deteriorating situation in Venezuela that has led to a 40 per cent decline in crude output in the country in two years. <PRODN-VE>
Nonetheless, further gains have been capped by rising US production as shale drillers ramp up activity in tandem with the rise in oil prices.
Surging US production, which hit 10.59 million bpd last week, has encouraged record-high US exports.