Chemical companies that supplied Agent Orange (search) to U.S. forces in Vietnam asked a federal judge on Monday to dismiss a lawsuit by Vietnamese citizens who say they were poisoned by the defoliant.

The plaintiffs allege that Monsanto Co. (MON), Dow Chemical Co. (DOW) and more than a dozen other firms violated international laws barring the use of poison and chemical weapons by producing Agent Orange, blamed for tumors, birth defects and other serious health problems. They are seeking potentially billions of dollars in damages.

Lawyers for the companies argued Monday that U.S. courts had no power to penalize the corporations for executing the orders of a president exercising his powers as commander in chief, and that international law generally exempted corporations, as opposed to individuals, from criminal and civil liability for alleged war crimes.

The Justice Department (search) also has sought dismissal of the lawsuit, arguing that opening the U.S. courts to former wartime enemies could threaten presidential power to wage war. The U.S. government has contended the claimed effects of Agent Orange are not supported by direct evidence.

U.S. District Judge Jack B. Weinstein seemed skeptical of the argument that corporations should enjoy such a broad exemption under international law, and he questioned whether presidential orders exempted the firms, noting the actions of German corporations during World War II.

"The fact that all power was centralized under Hitler did not permit all people operating under his orders to violate international law," Weinstein said.

However, Weinstein indicated skepticism about the plaintiffs' claims that the use of Agent Orange violated international law, saying it was far from clear whether agreements barring weapons such as poison gas applied to the case.

U.S. forces sprayed Agent Orange to clear jungles and destroy crops used by communist forces. More than 21 million gallons of the chemical, contaminated with toxic dioxins, were sprayed on Southeast Asia from 1962 to 1971.

Some 10,000 U.S. veterans receive medical disability benefits related to Agent Orange, which scientists have blamed for cancer, diabetes, birth defects and other problems among U.S. veterans and Vietnamese civilians.

Vietnam says up to a million people were affected by Agent Orange and other chemicals. A study last year showed high levels of dioxins persist in food samples.