Senator Barack Obama on Monday vowed to dispel smears over his "deep and abiding" love for America, but was locked in a fierce new row over Republican White House foe John McCain's military heroism.

The rival camps traded shots over the political minefields of patriotism and core American values, as the debate on who is best qualified to be commander-in-chief heated up ahead of 4 July Independence Day celebrations.

Obama meanwhile spoke by telephone with former president Bill Clinton, in another attempt to ease bruised feelings over the Illinois senator's rancorous tussle with Hillary Clinton for the Democratic presidential nomination.

Beset by claims he and wife Michelle shirk from love of country and heartland values, Obama spoke out in the symbolic venue of Independence, Missouri.

"The question who is or is not a patriot, all too often poisons our political debates in ways that divide us," Obama said, declaring America the greatest country in the world.

'Deep and abiding love'

"Throughout my life, I have always taken my deep and abiding love for this country as a given," said, Obama, a Christian, who has previously rejected unfounded allegations by shady opponents that he is a closet Muslim.

"At certain times over the last 16 months, I have found, for the first time, my patriotism challenged — at times as a result of my own carelessness, more often as a result of the desire by some to score political points and raise fears about who I am and what I stand for.

"I will never question the patriotism of others in this campaign, and I will not stand idly by when I hear others question mine."

"Surely we can agree that no party or political philosophy has a monopoly on patriotism," he said.

"Dissent does not make one unpatriotic," the Illinois senator (46) said against a backdrop of US flags.

Earlier this year, Michelle Obama sparked a row when she said her husband's campaign had left her "really proud" of her country for the first time in her adult life.

The candidate provided critics with ammunition to suggest that he was elitist and not in tune with basic US values, when he said in April that some working-class Americans were bitter and so turned to God and guns.

Clark ignites a firestorm

The Obama campaign faced another controversy after retired general Wesley Clark, a former Clinton supporter who now backs Obama, ignited a firestorm.

On CBS Sunday, Clark said he admired McCain's service, but suggested that record did not in itself qualify him to be president.

"I don't think riding in a fighter plane and getting shot down is a qualification to be president," he said.

On Monday Clark defended his own remarks but distanced them from the Obama camp.

"I wasn't representing the Obama campaign in anything I said yesterday about John McCain."

"I would never, never dis (disrespect) someone's service," Clark added.

"I came home from Vietnam on a stretcher. I was shot, I got four rounds. I was in three different hospitals and finally got home and eventually recovered and went on to serve. I think I know a little bit about what it's like to honor men and women who serve."

Obama's campaign released a statement repudiating Clark's Sunday remarks.

McCain (71) is offering himself as a veteran of national security and foreign policy questions who is best prepared to keep Americans safe.

A navy pilot shot down over North Vietnam and incarcerated in the infamous "Hanoi Hilton" jail, McCain said Clark's remarks were "unnecessary."

'I am proud of my record of service'

"I am proud of my record of service," McCain said, adding that it was up to Obama to decide whether to respond directly to Clark's comments.

His campaign cranked up the pressure with a conference call with reporters.

"I was utterly shocked when I saw this yesterday, that he would in a disrespectful way, attack one of his fellow career military officers," said Republican Senator John Warner, a respected military expert.

Bud McFarlane, a former national security advisor to ex-president Ronald Reagan, questioned Obama's credentials.

"Senator Obama, some day may have the good fortune to be able to engage with foreign leaders, or to have become as well read as John McCain in history and in national security affairs, but it doesn't exist right now."

Obama spokesperson Bill Burton meanwhile said the senator spoke with Bill Clinton by telephone.

"He has always believed that Bill Clinton is one of this nation's great leaders and most brilliant minds, and looks forward to seeing him on the campaign trail and receiving his counsel in the months to come," Burton said.