It was an "act of justice" to invade Iraq, US President George W Bush said on Thursday, adding he had no regrets despite the failure to find weapons of mass destruction there.

"Knowing what I knew then, and knowing what I know today, America did the right thing in Iraq," he told a handpicked crowd on the dock of Charleston Harbour.

Bush renewed his assertion that Iraq posed a direct threat to the US.

"When you're the commander-in-chief, you have to be willing to make the tough calls and to see your decisions through," he said, adding, "America is safer when our commitments are clear, our word is good, and our will is strong. And that is the only way I know how to lead."

Bush's speech was his most detailed defence of his decision since David Kay, who resigned last month as chief of the Iraq Survey Group, declared that the intelligence was wrong and Iraq had no unconventional weapons.

"The facts are becoming clearer," Bush said. "As the chief weapons inspector said, we have not yet found the stockpiles of weapons that we thought were there. Yet the Survey Group has uncovered some of what the dictator was up to."