US President George W Bush has said his rival in the presidential race, John Kerry, was the "more heroic" for having put himself "in harm's way" in Vietnam.
Recent TV ads alleged John Kerry lied about his Vietnam service
President Bush passed his military service as a fighter pilot in Texas.
Asked by US TV network NBC if he felt Mr Kerry and he "served on the same level of heroism", Mr Bush replied, "No, I don't."
Mr Kerry has accused President Bush of backing an ad campaign that accused him of lying about his valour in Vietnam.
A lawyer for the president's re-election team, Benjamin Ginsberg, resigned on Wednesday after admitting he advised the group of Vietnam veterans responsible for the ads.
President Bush has denied any links to the campaign and called for an end to unofficial negative advertising.
However, he has refused to specifically criticise the ads, which allege Mr Kerry did not deserve the medals he received for bravery in the Vietnam conflict.
Senator Kerry has made his service in Vietnam a cornerstone of an election campaign dominated by concerns over national security, traditionally seen as a Republican strength.
'Move beyond the past'
In his interview with NBC, President Bush said, "I think him [John Kerry] going to Vietnam was more heroic than my flying fighter jets. He was in harm's way and I wasn't."
He defended his own stint in the Texas Air National Guard, saying, "On the other hand, I served my country. Had my unit been called up, I would have gone."
Unofficial ads produced by a liberal group, funded by Democrat supporters, have alleged President Bush used family influence to avoid serving during the Vietnam war.
President Bush told the NBC network it was time to end the furore over the two candidates' Vietnam-era military records.
"I think that we ought to move beyond the past... The real question is who best to lead us forward," he said.