WASHINGTON (AFP) -- The United States took new steps to isolate Iran by freezing the sale of all F-14 fighter parts.

Iran bought 79 U.S.-built F-14 "Tomcat" fighters before the 1979 Islamic Revolution, and U.S. officials they could get spare parts for the planes through third parties.

The Defense Logistics Agency ordered the freeze January 26 "given the current situation in Iran," said agency spokesman Dawn Dearden on Tuesday.

Meanwhile, citing unnamed U.S. government officials, CNN claimed the Department of Defense is investigating whether a January 20 attack in the Iraqi city of Karbala was carried out by Iranians or by fighters trained by Iranians.

Attackers wearing uniforms resembling those of U.S. troops and traveling in vehicles often used by U.S. security contractors entered a secure U.S. compound, killing five U.S. soldiers.

"We believe it's possible the executors of the attack were Iranian or Iranian-trained," one of the officials told CNN.

"This was beyond what we have seen militias or foreign fighters do," the official said.

An unnamed senior Iraqi official told The New York Times in Baghdad that the attackers held forged U.S. identity cards, U.S.-style M-4 rifles and hurled stun grenades.

The Iraqi officials cautioned to the Times however they had not found a firm connection to Iran.