Republican Donald Trump on Wednesday invited Russia to dig up tens of thousands of "missing" emails from Hillary Clinton's time at the US State Department, vexing intelligence experts and prompting political foes to accuse him of dangerously urging a power abroad to spy on the United States.
"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump, the Republican presidential nominee, told reporters.
Clinton, a Democrat who faces Trump in the November 8 US presidential election, responded with a campaign statement saying he was posing a possible national security threat.
A spokesman for Trump, Jason Miller, later tried to tamp down the storm of protest, saying Trump did not urge Russia to hack Clinton's emails.
Trump was referring to a private email system Clinton kept while secretary of state from 2009-2013. She handed over thousands of emails in 2015 to US officials probing that system, but she did not release about 30,000 emails she said were personal and not work-related.
A Federal Bureau of Investigation probe of the issue found no basis for criminal charges, but FBI Director James Comey this month said there was evidence Clinton was "extremely careless" in her handling of classified information.
By focusing on Clinton's email drama, Trump drew attention away from the Democratic National Convention in Philadelphia where US President Barack Obama was due to speak on Wednesday night and Clinton was expected to accept the party's presidential nomination on Thursday.
At the same time, Trump at his news conference dismissed suggestions that WikiLeaks' release of embarrassing Democratic Party emails last week was engineered by Russia to meddle in the US election.
The Democratic Party chairwoman, Debbie Wasserman Schultz, resigned on Sunday after the leaked emails showed party leaders favoring Clinton over her rival, Senator Bernie Sanders, for the presidential nomination.
Cyber security experts and US officials have said there was evidence that Russia engineered the release of the sensitive party emails in order to influence the presidential election.
"It is so far-fetched, it's so ridiculous," Trump said of that notion on Wednesday. He suggested that China or some other party could be involved.
Russia has brushed aside suggestions it was involved. "I don't want to use four-letter words," Russian Foreign Minister Sergei Lavrov told reporters on Tuesday.
Trump, who has praised Russian President Vladimir Putin in the past, said on Wednesday he did not know the leader. He said his closest interaction with Russia was selling a Florida home to a Russian for more than he paid for it.
Miller, the Trump campaign spokesman, said on Twitter that Trump was not encouraging Russia to hack into Clinton's emails but rather was urging anyone who had her emails to hand them to the FBI.
On Twitter, Trump issued a similar appeal, saying if anyone had Clinton's emails, "perhaps they should share them with the FBI!"
But the Clinton campaign and intelligence experts said the comments raised questions about Trump's judgment.
"This is a national security issue now," Clinton campaign manager Robby Mook told reporters in Philadelphia.
"The idea that you would have any American calling for a foreign power to commit espionage in the United States for the purposes of somehow changing an election, we're now in national security space," he said.
Democrats at their convention on Wednesday night were likely to draw a contrast between their candidate's approach on national security and that of Trump, which her campaign characterised as dangerous.
Former CIA Director Leon Panetta said on CNN on Wednesday that Trump's comments were "beyond the pale" and showed that he was not qualified to be president. Michael Hayden, the CIA head under Republican President George W. Bush, told Buzzfeed News Trump's remarks were "problematic."
"Donald Trump's call on a foreign adversary, Russia, to illegally hack the email of his opponent for his own political benefit shows staggeringly poor judgment even for him," Adam Schiff, the top Democrat on the House of Representatives Intelligence Committee, said in a statement.
A US State Department spokesman declined to comment on Trump's remarks on Wednesday.
Trump's vice presidential running mate, Indiana Governor Mike Pence, took a different approach from the Republican nominee, saying he expected the FBI to 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," Pence said in a statement.
During the news conference at his Doral golf resort, which lasted more than 45 minutes, Trump also called Obama, a Democrat, the most "ignorant" president ever.
“I’m not an email person myself,” Trump said. “I think it can be hacked.”
Asked if he would recognise Crimea as Russian territory, Trump said he would be "looking into that." Crimea has long been part of Ukraine, but Russia annexed it in 2014 after pro-Russian separatists and special forces took over the region.