GENEVA - A United Nations human rights investigator on Friday accused U.S. and British forces in Iraq of breaching international law by depriving civilians of food and water in besieged cities as they try to flush out militants.

But the U.S. military denied the charge and said that while supplies were sometimes disrupted by combat, food was never deliberately withheld.

Jean Ziegler, a former Swiss sociology professor who is U.N. special rapporteur on the right to food, said the Geneva Conventions banned military forces from using "starvation of civilians as a method of warfare".

But he said that in Falluja, Tal Afar and Samarra, Iraqi and U.S.-led forces had cut off or restricted food and water to encourage residents to flee before assaults on entrenched Sunni insurgents over the past year.

"A drama is taking place in total silence in Iraq, where the coalition"s occupying forces are using hunger and deprivation of water as a weapon of war against the civilian population," Ziegler told a news briefing.

Two 1977 protocols to the 1949 Geneva Conventions, which lay down rules of conduct in armed conflicts, ban using deprivation of food or water as a weapon of war. They also prohibit destruction of food stocks or interruption of food supply lines.

Ziegler said he understood the military rationale of the coalition forces who were "facing such a horrible enemy -- these insurgents who do not respect any law of war and who use the civilian population of cities like Falluja or Tal Afar as human shields, who keep them as hostages".

But he said their actions were nevertheless a "flagrant violation of international humanitarian law".

Ziegler said he hoped the General Assembly would "condemn this strategy of the coalition forces" when he presents his report on the right to food in New York on October 27.

Lieutenant Colonel Steve Boylan, a spokesman for the U.S. military in Iraq, said Ziegler"s accusations were baseless.

"Any allegations of us withholding basic needs from the Iraqi people are false," he said.

"In conjunction with our combat operations, we take all precautions to ensure that the Iraqi people are taken care of, as does the Iraqi government," Boylan said.

"There have in the past ... been some supplies that have been delayed due to combat operations, but they were due to transit the area once it was deemed safe. It does not do relief supplies any good if you have them going into a firefight."

Ziegler said that he had been in touch with British authorities on the issue, and "a channel seems to be opening", but that attempts to start a dialogue with U.S. authorities had been fruitless.

Additional reporting by Luke Baker in Baghdad