Washington - Senator John McCain has taken a barely-veiled jab at President Donald Trump's avoidance of military service, sharply escalating their war of words.Republican McCain was interviewed by broadcaster CSPAN on the 50th anniversary of his being shot down over Vietnam, which led to his five-year stay as a prisoner in the infamous "Hanoi Hilton" prison."One aspect of the conflict, by the way, 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 that they had a bone spur," McCain said, apparently singling out a diagnosis that allowed Trump to be medically disqualified for service in 1968."That is wrong. That is wrong. If we are going to ask every American to serve, every American should serve," said McCain, himself an admiral's son who refused an offer of early release by his captors after being tortured.As a presidential candidate in 2015, Trump said McCain was a war hero only "because he was captured"."I like people that weren't captured," Trump said.'I'm being very nice'During the Vietnam War, Trump won four educational deferments before graduating from university.After receiving his diploma, he was again eligible for the draft. But a subsequent armed forces physical examination in 1968 found him medically unfit.The relationship between the pair - one a veteran lawmaker and war hero, the other a real estate tycoon and political neophyte before he assumed the presidency - has long been testy.But the acrimony has risen to a new degree of bitterness.Last week, McCain railed against the "America First" approach espoused by the Trump administration."To refuse the obligations of international leadership...for the sake of some half-baked, spurious nationalism cooked up by people who would rather find scapegoats than solve problems is as unpatriotic as an attachment to any other tired dogma of the past," said the 81-year-old, who was diagnosed with brain cancer in July.Trump lashed out on Tuesday, telling conservative radio host Chris Plante: "People have to be careful because at some point I fight back. I'm being very nice. I'm being very, very nice. But at some point, I fight back, and it won't be pretty."McCain has proved a thorn in Trump's side in recent months, twice helping to scupper Republican attempts to repeal a healthcare law that was a signature accomplishment of former president Barack Obama.