Oil for September delivery peaked at $45.75 a barrel on Thursday before settling back at $44.50, the highest closing price since the crude oil began trading on the exchange in 1983.
The price was 70 cents over oil's close on Wednesday. The previous record set earlier this week was $45.04.
American oil traders said fighting in the Iraqi city of Najaf and fears of attacks against Iraqi oil installations added to the jitters.
Also causing concern were hurricanes churning in the Gulf of Mexico and the Caribbean, which have forced the evacuation of oil rigs.
The unresolved conflict over the Russian oil company Yukos also caused nervousness.
US State Department deputy spokesman Adam Ereli said Washington had raised the issue with the government of Russia as recently as the end of July.
Strong worldwide demand for oil and limitations on production are raising fears over a shortage of supply with every new crisis, US oil experts say.