LOS ANGELES (AFP) -- Republican White House pick John McCain Wednesday broke with President George W. Bush on glaring disputes that soured relations with US allies, but said America had a moral duty to stay in Iraq.

The Arizona senator grabbed the spotlight as the Democratic race between Hillary Clinton and Barack Obama took another negative lurch, while the former first lady vowed to fight on all the way to the party's August convention.

"Our great power does not mean we can do whatever we want, whenever we want, nor should we assume we have all the wisdom and knowledge necessary to succeed," McCain said in a major foreign policy address here.

The Arizona senator heralded a hawkish approach to what he said was the "transcendent challenge of our time: the threat of radical Islamic terrorism," and mooted a robust defense of US interests against Russia and China.

McCain related his family's military heritage to frame his world-view as a "realistic idealist" and, in an apparent shot at Obama, warned "we cannot wish the world to be a better place than it is."

"I detest war. It might be the worst thing to befall human beings, but it is wretched beyond all description," the former Vietnam prisoner of war said.

"Whatever gains are secured, it is loss the veteran remembers. Only a fool or a fraud sentimentalizes the merciless reality of war," he told the Los Angeles World Affairs Council.

McCain carved out clear differences with Obama and Clinton, who have both vowed to end the unpopular Iraq war and bring US troops home if elected president in November.

"We have incurred a moral responsibility in Iraq. It would be an unconscionable act of betrayal, a stain on our character as a great nation, if we were to walk away from the Iraqi people," he said.

Such a "premature" and "irresponsible" US exit would consign Iraqis to horrendous violence, ethnic cleansing and possibly genocide, McCain said.

At the same time, McCain set a high bar for success in Iraq, one which Democrats will likely argue can no longer be attained.

"Success in Iraq and Afghanistan is the establishment of peaceful, stable prosperous, democratic states that pose no threat to neighbors and contribute to the defeat of terrorists.

"It is the triumph of religious tolerance over violent radicalism."

McCain warned withdrawal could precipitate regional chaos, boost Al-Qaeda and bolster Iran, drawing the United States into a wider, more difficult war.

Though strongly backing President George W. Bush's troop surge policy and continued US presence in Iraq, McCain sent strong signs of accommodation to traditional allies after rocky ties with the current Republican administration.

McCain, who endured torture in Vietnam, said he would close the Guantanamo Bay prison camp in Cuba, which has sparked outrage in Europe, and would outlaw torture and treat suspected terrorists humanely.

"The United States did not singlehandedly win the Cold War, the transatlantic alliance did, in concert with partners around the world.

"The bonds we share with Europe in terms of history, values and interests are unique. Americans should welcome the rise of a strong, confident European Union as we continue to support a strong NATO."

Again with an eye on major European powers, McCain called for a successor to the Kyoto accord on climate change, and a cap-and-trade system to cut greenhouse gas emissions.

McCain had harsh words for Moscow's "nuclear blackmail or cyber attacks" and he renewed his call for Russia to be ejected from the Group of Eight industrialized nations.

Clinton meanwhile promised no surrender in her bruising battle against Obama for the Democrats' White House nomination.

In an interview with Time magazine published Wednesday, the New York senator said even pledged delegates would be up for grabs as she vies to overhaul Obama's seemingly impregnable lead in the 10 remaining nominating contests.

"We talk a lot about so-called pledged delegates, but every delegate is expected to exercise independent judgment," she said, staking out a position on elected convention representatives that is hotly contested by the Obama camp.

"And, you know, I'm just going to do the best I can in the next 10 contests to make my case to the voters in those elections, and then we'll see where we are," she said.