Brent crude futures lost 9 cents to $74.42 a barrel by 1:06 p.m. EDT (1706 GMT). The international benchmark reached $74.73 a barrel on Tuesday, highest since Nov. 1.
US West Texas Intermediate crude futures fell 49 cents to $65.81 a barrel. The contract hit $66.60 a barrel on Tuesday, the highest since Oct. 31.
US crude inventories rose 5.5 million barrels last week, the Energy Information Administration said, far more than analysts' forecast of an increase of 1.3 million barrels. However, gasoline stocks fell by 2.1 million barrels, a larger-than-anticipated drop.
"What we're looking at is a headline number bearish on crude but supported somewhat by the gasoline number," said Phil Flynn, an analyst at Price Futures Group in Chicago.
"Because of the sanctions that are coming down on Iran and the fact that there's going to be no waivers, it makes this number look more bullish."
Crude futures and prices for spot delivery rallied after the United States said on Monday it would end all exemptions for sanctions against Iran, demanding countries halt oil imports from Tehran from May or face punitive action. The move raised worries about tighter global oil supplies.
The United States must be prepared for consequences if it tries to stop Iran from selling oil and using the Strait of Hormuz, Iran's foreign minister, Mohammad Javad Zarif, warned on Wednesday.
China, Iran's biggest oil customer, has formally complained about the move.
The spot price surge put the Brent forward curve into steep backwardation, in which prices for later delivery are cheaper than for prompt dispatch.
The United States has said it saw Saudi Arabia as a partner to balance oil markets.
Signaling no immediate action to counteract missing Iranian barrels, Saudi Energy Minister Khalid al-Falih said on Wednesday that his country's production in May would not vary greatly from previous months.
He added that Saudi Arabia aimed to stick to its output quota fixed in a deal by the Organization of the Petroleum Exporting Countries, Russia and others, but that June numbers would be determined depending on customers' needs.
"Saudi has created ~500 kbd of 'swing capacity' within its OPEC+ agreement allocation, running at ~9.8 mbd versus ~10.3 mbd cap," Paul Sankey, an energy analyst at Mizuho, wrote in a note.
"If the market starts to appear too tight with a reduction in Iranian exports, Saudi has some capacity to ease the impact without breaking its OPEC+ pledge and endangering OPEC+ group commitment."
Meanwhile, crude output in the United States, which turned into the world's top producer last year, last week edged back to its record high at 12.2 million barrels per day, EIA data showed.