U.S. officials are downplaying media reports that Washington is investigating possible Iranian involvement in an attack in Iraq that killed five U.S. soldiers.

At the State Department, a senior U.S. official said he does not know anything to substantiate the conclusion that Iran was involved, and called the idea a "supposition."

White House spokesman Tony Snow said he would not comment on what it called "speculation" about Iranian involvement in the attack last Saturday in Karbala.

The New York Times and CNN had reported that the Defense Department is trying to determine whether Iranians or Iranian-trained operatives carried out the attack.

The U.S. military has said the attack was well coordinated, with assailants dressed in U.S. military-style uniforms and driving vehicles similar to those used by U.S. troops.