WASHINGTON —  Sen. Lindsey Graham asked Attorney General John Ashcroft Monday to provide him with a legal assessment of those Americans headed to or already in Iraq to offer themselves as "human shields."

Graham, who has been vehement in his opposition to Americans who go to Iraq and calls them "treasonous," said he believes the "full force of the law should be applied to those American citizens who give aid or comfort to our enemies."

"It is my opinion that any American who voluntarily engages in conduct to impede a potential American military operation, and who thereby endangers the lives of our nation's men and women in uniform, is participating in a program designed to weaken the power of the United States to wage war successfully. I strongly believe efforts to impede a potential military operation against Iraq should be strongly dealt with and I am seeking your assistance in this matter," Graham, R-S.C., wrote in a letter to Ashcroft on Monday.

"Our constitution and federal legal structure do not allow Americans to actively aid nations or groups engaged in hostilities with the United States. The recent conviction of American John Walker Lindh for his assistance with Al Qaeda is one such example."

Graham asked Ashcroft if the Justice Department has considered formally notifying Americans engaged in such conduct of the legal risks they are about to assume. In January, Iraq announced that it was bringing in several groups from Arab, European nations and the United States to act as human shields.

But this weekend, several British war protesters, who traveled from London to Baghdad to act as human shields, returned to England, saying that they feared for their safety. Supporters of the movement said they thought they could prevent a war if several thousand people acted as shields but only a few dozen showed up.

In addition, protesters said the Iraqi government was limiting their access to sites they wanted to shield, such as hospitals, but in the meantime was using them as propaganda by giving them accommodations, transportation and arranging news conferences for them.

The United States has previously said that using civilians as human shields is a war crime. The government also said that the civilians' safety can't be ensured.

Fox News' Jim Mills contributed to this report.