U.S. media reports say the United States is investigating possible Iranian involvement in a recent attack in the Iraqi city of Karbala, in which five American soldiers were killed.

The reports in the New York Times and CNN television quote unnamed U.S. officials as saying the Defense Department is trying to determine whether Iranians or Iranian-trained operatives carried out the attack on a U.S. military compound last Saturday.

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.

The White House said Wednesday that it would not comment on what it called "speculation" about Iranian involvement in the Karbala attack. But spokesman Tony Snow repeated U.S. warnings to respond "forcefully" to anyone trying to kill U.S. troops in Iraq or destabilize the country.

Some information for this report was provided by AFP.