Donald Trump has raised new questions about his commitment to the defence of NATO allies on the eve of his acceptance of the Republican presidential nomination.

In a 45-minute interview with The New York Times, Mr Trump also expressed little willingness to speak out against purges or civil rights crackdowns by authoritarian allies like Turkey.

"I don't think we have the right to lecture," he said.

"Look at what is happening in our country. How are we going to lecture when people are shooting policemen in cold blood?"

Mr Trump also re-emphasised the hardline nationalist approach that he has taken during his campaign, describing how he would force allies to shoulder defence costs that the United States has borne for decades.

Asked about Russian activities that have alarmed the Baltics — NATO's newest members — Mr Trump said that if Russia attacked them, he would decide whether to come to their aid only after reviewing whether those nations "have fulfilled their obligations to us".

"If they fulfil their obligations to us, the answer is yes," he said.

A cornerstone of the 28-member trans-Atlantic alliance is its Article 5 commitment that an attack on one member state is an attack on all, a pledge invoked after the September 11, 2001 attacks on the United States.

Mr Trump, who said he would press the theme of America First at his address on Thursday night to the Republican National Convention, said allies would adjust to his approach.

"I would prefer to be able to continue [existing agreements]," he told the Times, but only if allies stopped taking advantage of an era of American largesse that was no longer affordable.

AFP