But McCain campaign advisers seized on the remark as evidence to support their contention that while Obama preaches a civil message, he is willing to engage in cutthroat tactics.

They organized a conference call with some of McCain's colleagues in the military to defend his honor.

"They're playing politics," said Orson Swindle, who was in the POW camp with McCain. "It's not good. Senator Obama apparently is permitting it, and that's very disappointing."

McCain said the controversy should not distract from the important election issues.

"If that's the kind of campaign that Senator Obama and his surrogates and his supporters want to engage, I understand that. But it doesn't reduce the price of a gallon of gas by one penny. It doesn't achieve our energy independence ... (and) it doesn't help an American stay in their home," McCain told reporters in Harrisburg, Pennsylvania.

Obama, an Illinois senator who would be the first black US president, will face McCain, the presumptive Republican nominee, in November's election for the White House.

Obama has battled persistent criticism about his failure to wear a flag pin on his lapel, viewed as a symbol of patriotism for some US politicians. He often wears one now.

He also has been the target of Internet rumors about his willingness to recite the Pledge of Allegiance, and his wife, Michelle, has been criticized for a remark she said was taken out of context about feeling proud of her country.