INDEPENDENCE, Mo. (Reuters) - Kofi Annan, in his last major speech as UN secretary general, urged the United States on Monday to shun go-it-alone diplomacy and collaborate on its world challenges, including the Iraq war.

In a farewell address delivered at Harry Truman's presidential library in Independence, Missouri, Annan praised the 33rd U.S. president's legacy, and quoted Truman in cautioning that "no nation can make itself secure by seeking supremacy over all others."

Truman was a strong backer of the United Nations and helped found the world body.

Annan, who steps down at the end of the month, to be succeeded by Ban Ki-Moon of South Korea, said, "We need U.S. leadership; we have lots of problems around the world ... and we require the natural leadership role the U.S. played in the past and can play today.

"None of our global institutions can accomplish much when the U.S. remains aloof. But when it is fully engaged, the sky's the limit," he said.

In Washington, State Department spokesman Sean McCormack said the outgoing U.N. chief 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. It's not that person's job," McCormack said. Republican Rep.

Henry Hyde of Illinois, the retiring chairman of the House International Relations Committee, said Annan failed to mention "the rampant financial and moral mismanagement at the United Nations" and called his remarks "a classic case of misdirection aimed at the United States."

During his two five-year terms as UN leader, Annan has tangled often with President George W. Bush's administration, particularly over the invasion of Iraq in March 2003, launched without a green light from the UN Security Council.

"When power, especially military force, is used, the world will consider it legitimate only when convinced that it is being used for the right purpose -- for broadly shared aims -- in accordance with broadly accepted norms," Annan said.

In response to a question on how to end the war in Iraq, Annan said the United States needed to work with other countries, including Iran and Syria, to foster a "sharing" of political power and oil revenues within Iraq's Sunni and Shiite factions. "If you make them responsible and pull them into work with you, I think it will be in everyone's interests," he said.

"Getting Iraq right is not only in the interests of the U.S. and the broad international community but even more so for the countries in the region."

Annan renewed a call to expand the 15-nation Security Council and took a dig at U.S. opposition to a plan to add 10 seats.

Bush administration officials have argued Washington should use the United Nations only to serve its national interests. But Annan said it was crucial to organize UN bodies "in a fair and democratic way, giving the poor and the weak some influence over the actions of the rich and the strong."

"It is only through multilateral institutions that states can hold each other to account," he said.

The United States has historically been a leader in human rights, noted Annan.

"When it appears to abandon its own ideals and objectives, its friends abroad are naturally troubled and confused," he said in an apparent reference to charges of abuse at U.S. prisons in Guantanamo Bay, Cuba, and Iraq's Abu Ghraib.

Truman, who ordered two atomic bombs dropped on Japan in 1945, learned from that experience that security from then on "must be collective and indivisible," Annan said.

"All civilization is at stake, and we can save it only if all peoples join together in the task," Annan said.

"You Americans did so much, in the last century, to build an effective multilateral system, with the United Nations at its heart. Do you need it less today, and does it need you less, than 60 years ago?"