Brent North Sea crude for delivery in November slid USD 1.59 to stand at USD 109.01 a barrel in late London deals.
New York's main contract, West Texas Intermediate for October shed USD 1.04 to USD 107.03.
"Crude oil prices after rising strongly... Have started to slip back on comments made by Iranian president Rowhani," said Michael Hewson, senior market analyst at trading group CMC Markets.
Iran's new President Hassan Rowhani has described Israel as an "occupier" that has brought instability to the Middle East but said the Islamic republic did not seek war with any country.
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In the second part of an exclusive interview with US broadcaster NBC aired today, Rowhani also deflected a question on whether, like his predecessor Mahmoud Ahmadinejad, he believed the Holocaust was a myth.
But he said Tehran was not seeking war but rather peace in the region when specifically asked about archfoe Israel.
Oil prices had risen strongly yesterday and earlier today after the Fed confounded market expectations with its decision to refrain from tapering its USD 85-billion-a-month bond-buying programme.
"The Fed surprised the markets yesterday after reporting that there will be no tapering in its bond buying programme," said senior energy analyst Myrto Sokou at the Sucden brokerage in London.
"Global equity markets and commodities rallied on the unexpected Fed decision, while crude oil prices rebounded strongly."
Markets had expected the US central bank's policy setting Federal Open Market Committee (FOMC) to scale down its USD 85 billion a month bond-buying spree, known as quantitative easing (QE).
But the FOMC said that although the economy appears to be holding up amid government spending cuts, it "decided to await more evidence that progress will be sustained" before deciding to scale down the stimulus package.