Donald Trump on Thursday said that he was speaking in jest when he made an open request to Russia to hack into Hillary Clinton's emails.

"Of course I was being sarcastic," he said on news show Fox and Friends.

The Republican presidential nominee reiterated that he did not know who was responsible for the hack.

"They have no idea if it's Russia, if it's China, if it's somebody else," he said. "Who knows who it is?"

Just one day before, after denying he had colluded with Russian President Vladimir Putin to leak Democratic National Committee emails, Trump expressly asked Russia to find "missing" emails belonging to Hillary Clinton.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," he told reporters on Wednesday in Doral, Florida. "I think you will probably be rewarded mightily by our press. Let's see if that happens. That will be next."

The DNC, cybersecurity experts, and US officials have said Russian state hackers were most likely behind the hacking of thousands of DNC emails, which were uploaded to WikiLeaks on Friday. The FBI is now investigating the incident.

The Republican nominee for president denied any ties to Putin or any role in the leak, which Democrats have suggested was done to boost Trump's campaign.

"I'm not going to tell Putin what to do. Why should I tell Putin what to do?" he said.

Trump denied there was any evidence that Russia was behind the attack, but also said the hacking showed a lack of respect of the US.

"But if it is Russia, it's really bad for a different reason, because it shows how little respect they have for our country when they would hack into a major party and get everything," he said, before immediately asking Russia to hack Clinton's emails.

Later in the day, Trump told supporters at a town hall in Pennsylvania he questioned how a Clinton campaign official could be sure Russia was behind the hack.

"Might be a 400-pound person sitting in bed," he said. "Might be. Some of the greatest hackers of all time. But he said Russia. And then he said Trump."

Trump added that he hadn't done anything related to the leak of the emails.

"I wish I had that power," Trump said. "Man, that would be power."