Donald Trump once again refused to condemn Russian President Vladimir Putin in the final presidential debate – saying Putin has 'no respect' for his rival, Hillary Clinton.

An angry debate had some of its harshest and most rapid-fire charges yet between the two on the same stage when Wallace asked about Russia, a strategic rival to the United States.

Hacking that the U.S. government says Russia has been behind has roiled the campaign, exposing awkward emails with the release of thousands of messages by Clinton campaign chair John Podesta in recent days.

Trump once again refused to criticize Russian President Vladimir Putin – who he said wasn't his 'best friend.'

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'I don't know Putin. He said nice things about me. If we got along well, that would be good,' Trump said.

Trump, who has previously said Putin is a stronger leader than President Obama, said, 'He has no respect for her, he has no respect for our president, and I'll tell you what, we're in very serious trouble.'

He continued: 'Putin, from everything I see has no respect for this person,' driving home the point.

'Well that's because he'd rather have a puppet as president of the United States,' Clinton countered.

A supporter of hers, former CIA deputy director Michael Morell, has said Trump may be an 'unwitting agent' of the Russian Federation, citing a series of campaign ties and policy positions.

'You're the puppet!' shot back Trump.

Clinton countered that Trump was willing to 'spout the Putin line' and implement his policy 'wish list' in Syria and other countries.

'You continue to get help from him because he has a very clear favorite in this race,' Clinton said.

Clinton then pivoted to the repeated hacks on Democrats, including her campaign chair and on the Democratic Party.

'This is such an unprecedented situation,' Clinton continued.

Attack: The exchange on Russia began in a discussion of immigration, veered to Wikileaks and went on to cover hacking

'Hillary, you have no idea,' Trump interjected.

'She doesn't like Putin because Putin has outsmarted her at every step of the way,' Trump said.

Trump at one point in the campaign invited Russia to hack Clinton's emails, but said he was being sarcastic. During the debate he brought up some of the emails, which have been dumped on WikiLeaks.

'John Podesta said some horrible things about you, and boy was he right,' Trump said.

Wallace asked Trump point blank whether he would condemn Russian hacking – and Trump did, though in language that was vague.

'By Russia or anybody else,' Trump responded – keeping alive the possibility that someone else was behind the hacking. 'Of course I condone,' he said – leaving out a direct object in his sentence instead of saying he condoned Russian hacking.

Later in the debate, Trump responded to Clinton's charge that he put NATO allies at risk with past comments that he might not jump to defend those who are owing in their dues to defend the alliance.