Republican senator John McCain has declined to label Donald Trump a "draft dodger", even as he renewed his veiled criticism of the medical deferments that kept the US President from serving in the Vietnam War.

Key points: McCain says it's wrong the poorest had to serve while the wealthy got bone spurs

McCain says it's wrong the poorest had to serve while the wealthy got bone spurs He confirms to ABC America that the comments were directed at Trump

He confirms to ABC America that the comments were directed at Trump McCain has had a bumpy relationship with Trump, and now says they barely talk

"I don't consider him so much a draft dodger as I feel that the system was so wrong that certain Americans could evade their responsibilities to serve the country," Senator McCain said on ABC America.

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Senator McCain, a former Navy pilot and prisoner of war, was being pressed about comments in a C-SPAN interview aired yesterday, in which he lamented that the military drafted the poorest Americans while the wealthy could bypass the process.

"It was a tumultuous time and most of it was bred by the conflict," Senator McCain told C-SPAN.

"And one aspect of the conflict that I will never, ever countenance, is that we drafted the lowest income level of America and the highest income level found a doctor that would say they had a bone spur.

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"That is wrong. That is wrong.

"If we're gonna ask every American to serve, every American should serve."

One of Mr Trump's five draft deferments came as a result of a physician's letter stating he suffered from bone spurs in his feet.

Mr Trump's presidential campaign described the issue as a temporary problem.

Senator McCain, meanwhile, spent more than five years as a prisoner of war after his plane was shot down over North Vietnam in 1967.

Yet during last year's presidential campaign, Mr Trump said Senator McCain was not a war hero because "I like people who weren't captured".

But Senator McCain left little doubt during Monday's interview that he had been referring to the US President in his C-SPAN comments.

When one of the hosts remarked that people thought he was talking about Mr Trump because the President had sought a medical deferment, Senator McCain interjected, "More than once, yes."

Senator McCain was also asked to describe his relationship with the President, to which he said he had "almost none".

'I have faced tougher adversaries': McCain

The six-term Arizona lawmaker, battling brain cancer at age 81, made his appearance on ABC America's The View in honour of his daughter Meghan McCain's birthday — she recently joined the daytime talk show as one of its panel of co-hosts.

The White House has declined to comment on Senator McCain's remarks on C-SPAN.

The tacit criticism from Senator McCain reflected the ongoing tension between Mr Trump and the Republican senator, which began during last year's campaign and has flared on and off.

Mr Trump responded furiously when Senator McCain's "no" vote sunk Senate efforts to repeal and replace "Obamacare" earlier this year.

And last week, in a speech in Philadelphia, Senator McCain questioned "half-baked, spurious nationalism" in America's foreign policy, after which Mr Trump lashed out, insisting he would fight back and "it won't be pretty".

That prompted Senator McCain to retort: "I have faced tougher adversaries."

AP