Republican presidential candidate Donald Trump called on Russia to hack into rival Hillary Clinton's email and publicly release her private conversations.
Ms Clinton did not hand over a number of emails to US officials as part of a probe into the use of her private email system, saying they were private.
"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Mr Trump told reporters on Wednesday (US time).
He repeated the controversial call on Twitter.
Hillary for America spokesperson Jake Sullivan said Mr Trump's appeal to Russia was encouraging "a foreign power to conduct espionage".
"This has to be the first time that a major presidential candidate has actively encouraged a foreign power to conduct espionage against his political opponent," he said.
"That's not hyperbole, those are just the facts. This has gone from being a matter of curiosity, and a matter of politics, to being a national security issue."
Mr Trump also said Russian President Vladimir Putin calls outgoing US President Barack Obama "the N word".
"Number one, he doesn't like him," says Mr Trump. "Number two, he doesn't respect him… He will respect me."
CNN reports there is no evidence to back up this claim.
Separately, the Republican presidential nominee dismissed any suggestion that Russia was behind the theft and release of embarrassing Democratic Party emails made public last week.
"It is so far-fetched, it's so ridiculous," Mr Trump said, suggesting that China or some other party could be involved.
But cyber security experts and US officials have said there was evidence that Russia engineered the release of sensitive Democratic Party emails in order to influence the presidential election.
Ms Clinton kept a private system for her emails at her New York home while serving as secretary of state from 2009-2013.
Those emails were the subject of a Federal Bureau of Investigation probe that found no basis for criminal charges despite what FBI director James Comey called evidence Clinton was "extremely careless" in her handling of classified information.
Democrats were embarrassed last week by the WikiLeaks release of internal party emails showing party leaders had favoured Ms Clinton over her party rival, Senator Bernie Sanders, in the race for the presidential nomination. The party chairwoman, Debbie Wasserman Schultz, resigned on Sunday over the affair.
Trump's running mate, Mike Pence, took a different approach from Mr Trump to the release of the Democratic Party emails, saying the FBI would get to the bottom of the matter.
"If it is Russia, and they are interfering in our elections, I can assure you both parties and the United States government will ensure there are serious consequences," he said in a statement.
Russian Foreign Minister Sergei Lavrov on Tuesday brushed aside accusations that Moscow was behind the hacking of Democratic Party emails.
"I don't want to use four-letter words," Lavrov told reporters, when asked whether Russia was responsible for the email hacking.
Reuters / Newshub.