Donald Trump has “actively encouraged” foreign powers to hack his presidential rival Hillary Clinton, her camp says.
Mrs Clinton did not hand over 30,000 emails as part of an investigation into her private email server as they contained private details.
“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing,” Mr Trump said on Wednesday.
“I think you’ll be rewarded mightily by our press.”
The emails would contain some “beauties”, he said. Soon after, he wrote on Twitter that if anyone had the emails, they should hand them over to the FBI.
His appeal comes as Russia stands accused of hacking emails from the Democratic National Committee (DNC) for Mr Trump’s benefit. Both Russia and Mr Trump deny the allegation.
“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,” Jake Sullivan, Mrs Clinton’s senior policy adviser, said.
“This has gone from being a matter of curiosity, and a matter of politics, to being a national security issue.”
In a statement released within an hour of Mr Trump’s comments, his vice-presidential candidate Mike Pence said there would be “serious consequences” if the FBI could prove Russia was attempting to interfere with the election.
The emails were leaked to the Wikileaks organisation and published on Friday.
The FBI is continuing its investigation into the leak, which included emails that showed DNC officials, who are supposed to remain neutral, favoured Mrs Clinton, and derided her Democratic rival Bernie Sanders.
Mrs Clinton’s campaign manager, Robby Mook, said Russia carried out the hack to weaken the Democrats and help Mr Trump.
In an interview with NBC News on Wednesday, President Obama – whose own government has been accused of carrying out hacks on other governments – refused to rule out the possibility Russia was responsible, adding: “What we do know is that the Russians hack our systems. Not just government systems, but private systems.”