Donald Trump is continuing to stoke his feud with veteran Republican senator John McCain, warning the Arizona Republican he would “fight back” against criticism.

Among Republicans who have criticized the president, Senator McCain has emerged as a particularly vocal detractor. He also earned Mr Trump’s animus by casting a key vote against the president’s push to repeal Barack Obama’s healthcare law.

This week he issued a stinging rebuke of Mr Trump’s foreign policy - which has been animated by skepticism of international agreements - in a speech that slammed abdicating “the obligations of international leadership” because of “some half-baked, spurious nationalism by people who would rather find scapegoats than solve problems”.

Receiving the Liberty Medal in Philadelphia on Monday, the six-term senator from Arizona warned against the US surrendering its international leadership.

He said: “To fear the world we have organised and led for three-quarters of a century, to abandon the ideals we have advanced around the globe, to refuse the obligations of international leadership and our duty to remain the last best hope of Earth 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 that Americans consigned to the ash heap of history."

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In a radio interview with WMAL on Tuesday, Mr Trump referenced Mr McCain’s “shocker” of a vote against healthcare, in response to which radio host Chris Plante referenced Mr McCain’s speech in saying the senator “was taking shots at you yesterday”.

“People have to be careful, because at some point I fight back, and it won’t be pretty”, Mr Trump replied.

According to CNN, Mr McCain responded by saying “I’ve faced far greater challenges than this”. The six-term senator spent years as a prisoner of war in Vietnam and was recently diagnosed with brain cancer.

The two men have an uneasy history. During the presidential campaign Mr Trump spurred outrage for saying of Mr McCain “I like people who weren't captured”, and the two hold starkly different visions of America's place in the world.

But both Republican senators from Arizona have clashed with the president. Senator Jeff Flake wrote a book lambasting the president, which has led Mr Trump to publicly denounce Mr Flake and back an election challenge from another Republican.