U.S. West Texas Intermediate (WTI) crude futures were trading 15 cents lower at $68.28 per barrel by 0719 GMT.
Crude has rallied about 15 percent so far this year as OPEC and allied producers crimp a global glut and geopolitical risks multiplied in the Mideast region that's home to nearly half the world's oil. The extent of the rally would have been significantly weaker if not for recent tightness in the market.
The price of a barrel of Brent crude, the worldwide benchmark contract, slipped to $72.91 per barrel, down about 45 cents.
Technical analysis from Reuters' Wang Tao showed the market may retest a price support level at $72.39 per barrel after peaking around a resistance at $75.45.
Iran is the world's fifth-largest oil producer, and a third of all global oil tankers travel through the Strait of Hormuz, a narrow passageway controlled by Iran, making any economic or military standoff especially worrisome for the global oil supply.
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Expectations that the USA will pull out of the Iran deal and refrain from extending sanctions relief are "keeping both crude markers near three-year peaks", says Stephen Brennock of London brokerage PVM Oil Associates; Pres. Trump's self-imposed deadline for a decision on Iran is May 12.
Markets are increasingly expecting President Trump to pull out of the Iran nuclear deal negotiated by the Obama administration. "Investors are anxious about supplies after Iran took a tough stance in its response to the United States", said Wang Xiao, head of crude research with Guotai Junan Futures.
The US now produces more crude oil than top exporter Saudi Arabia.
Never mind that Trump's own secretary of Defense, James N. Mattis, told Congress last week that the agreement provides for "pretty robust" oversight of Iran's nuclear activities. But they have stopped short of saying the entire agreement would be void if the USA pulled out. But they have also started work on protecting EU-Iranian business ties if Trump makes good on his threat to withdraw.
That was days before Iran accused President Donald Trump of "bullying", while the US signaled it's preparing to pull out of a nuclear accord that's allowed OPEC's third-largest producer to export more crude.