Kofi Annan has made his final speech as UN secretary general, calling on the US not to lose sight of its core principles in its fight on terror.
"No nation can make itself secure by seeking supremacy over others," Mr Annan said, urging the US to respect human rights in its "war on terror".
Mr Annan said states had to be accountable and the UN was the only body where this could be assured.
The speech has been interpreted as a sharp rebuke of President Bush.
In Washington, State Department spokesman Sean McCormack said Mr Annan was entitled to his opinions.
"There's no secretary-general of the United Nations that's going to be in lock-step with the United States or any other country with regard to its policies," he said.
"It's not that person's job."
Abuse of power
Our correspondent in Missouri, Jonathan Beale, said Mr Annan again raised objections to the Iraq war, a war he has already condemned as illegal.
"When power, especially military force, is used, the world at large will consider it legitimate only when convinced that it Is being used for the right purpose - for broadly shared aims," he said.
KOFI ANNAN'S FIVE LESSONS
The rule of law
Mr Annan, who has led the UN since 1997, will be succeeded by South Korea's Ban Ki-moon on 1 January.
He delivered his speech at the library of late US President Harry Truman in the Missouri city of Independence and spoke of five lessons that he had learnt during his time at the head of the UN.
In the address, he urged the US to embrace its natural and historical role as responsible global leader and warned that no nation can make itself secure by seeking supremacy over others.
He praised the US for being historically "in the vanguard of the global human rights movement".
But he also said that "that lead can only be maintained if America remains true to its principles - including in the struggle against terrorism".
"When it appears to abandon its own ideals and objectives, its friends abroad are naturally troubled and confused."
Mr Annan also stressed that Washington's current position in the world gives it "a priceless opportunity" to entrench the principles of democracy at a global level.
But he stressed that this was no clash between Western and Eastern civilisations.
"All civilisation is at stake, and we can save it only if all peoples join together in the task."
Mr Annan's speech is also highly symbolic, our correspondent says, as the outgoing UN head has chosen the venue deliberately - the library of President Truman.
President Truman was an early champion of the UN - a contrast to Mr Bush, who has been one of its harshest critics, our correspondent says.
During the speech, Mr Annan repeatedly quoted the words and philosophy that informed Mr Truman's politics.
He ended with an appeal for a shift in US policy, saying "In order to function effectively, the system still cries out for far-sighted leadership in the Truman tradition."
"I hope and pray that American leaders of today and tomorrow will provide it."
Born in Ghana in 1938, Mr Annan has led the UN since 1997 and, in 2001, he and the UN received the Nobel Peace Prize.