Donald Trump has challenged Russia to find "missing" emails belonging to Hillary Clinton.
Thousands of emails that Hillary Clinton did not hand over to US officials as part of a probe into the use of her private email server are still to emerge.
Cyber-security experts and US officials have blamed Russian hackers for the hacking of thousands of the Democratic National Committee's emails, which were released by the Wikileaks group last week.
"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," he said.
"I think you will probably be rewarded mightily by our press."
Mr Trump predicted that there would be some "beauties" among them.
Our Washington Correspondent Robert Moore said Trump's were "bizarre" even by his standards.
Clinton made history when she became the first woman to be officially nominated for president by a major US party.
He repeated the challenge later on Twitter.
A spokesperson for Hillary Clinton told MSNBC that Trump "is now openly inviting Russia to engage in cyber-attacks against the United States. This is a bridge too far."
"It is amazing to me that Donald Trump apologises for Vladimir Putin's Russia at every turn and is now recklessly inviting him to engage in cyber-attacks against our own home country," the spokesperson added.
Mr Trump's senior communications adviser told NBC that Trump wasn't "calling on anyone to intervene or anything of the sort" but pushing anyone who has them to hand them over to the authorities.
When questioned about Russia's possible involvement with the Democratic party leak, Trump said it was "far-fetched" to suggest it might have been sanctioned by President Vladimir Putin to help his presidential campaign.
The emails showed high-ranking party officials favouring Clinton's bid for the party's White House nomination over Bernie Sanders'.
"It is so far-fetched, it's so ridiculous. Honestly, I wish I had that power. I'd love to have that power," Trump said.