In sharp contrast to his defence minister's remarks, Shinzo Abe, the Japanese prime minister, praised the US president's proposal to add some 21,000 troops to US forces in Iraq.
"The increase in troops demonstrates America's strong determination to bring stability and recovery to Iraq," Abe said after George Bush's State of the Union address on Tuesday night.
"I hope the United States' new measures are effective."
The rare public criticism from one of Washington's closest allies was in response to new plans announced by Bush to boost troop numbers in Iraq.
Japan, which withdrew 600 troops in June but provides air support in Iraq, said it would not rush into extending that commitment, set to end in July.
"Bush's strategy has failed totally in Iraq"
Dimos, Hania-Crete, Greece
Send us your views
Bush, in his address, implored the US Congress to support his Iraq plan to send more troops to Iraq.
Kyuma, whose ministry was upgraded from an agency to a fully fledged ministry in January, said on Wednesday that Bush was wrong to invade Iraq "based on an assumption that weapons of mass destruction existed".
He said Junichiro Koizumi, the former prime minister, had supported the war and that had made it hard to object to the deployment, but Japan's stance had been one of understanding, not agreement.
"At the time I said Japan's situation was such that it should limit its stance to one of understanding because it couldn't oppose the decision to go" to war, he said. "My opinion hasn't changed since then."
Whether Japan decides to extend the humanitarian mission and support the international reconstruction efforts in Iraq, he said, "will depend on the policies of the United Nations and the US".
Kyuma also brushed off criticism of Japan's growing military role and concerns that the newly elevated defence ministry might portend a Japanese return to militarism.
"There is no need for concerns that the upgrading of the defence ministry is a step toward becoming a military power or expanding Japan's defence spending," he said.
Our defence-focused security policy, as well as peacekeeping and non-nuclear principles, remain unchanged."