"You well know that what explains our country's disagreement with the way the war was carried out was that it clearly did not at that time abide by international law, and there was not a clear request from the United States to start that action," French Foreign Minister Michel Barnier said on Friday.
That was "traditionally" France's view from the start, he added.
"We have always considered that international law constitutes the framework for any action, notably against terrorism or for stability in the world," he said.
Barnier's comments added fuel to a debate over the legitimacy of the US-British invasion of Iraq that promises to loom large at the UN in New York next week when world leaders and ministers gather for the world body's 59th General Assembly.
Annan threw the spotlight back on the issue and tore the skin off old transatlantic wounds when he told BBC radio on Wednesday that the US had failed to seek a needed second resolution before launching the war in March 2003.
"I've indicated that it was not in conformity with the UN charter from our point of view, and from the charter point of view it was illegal," Annan said.
Washington hit back on Thursday by claiming it considered that a previous UN resolution passed four months prior to the conflict gave it sufficient authority to wage its action because Iraq had refused to surrender suspected stockpiles of weapons of mass destruction.
After 19 months of US troop deployment in Iraq, no such weapons have been found by US weapons inspectors scouring the war-ravaged country.