WASHINGTON - Republican Donald Trump appealed to Russia on Wednesday to uncover and release thousands of emails Hillary Clinton did not hand over to US officials who investigated her use of a private email system when she was secretary of state.

Clinton, a Democrat who faces Trump in the Nov 8 White House election, responded with a campaign statement accusing him of posing a possible national security threat by urging Russia to commit espionage and influence the vote.

"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 has said the emails she did not turn over were private.

Clinton kept a private system for her emails at her New York home while she was at the State Department from 2009-2013. She is due to accept the presidential nomination on Thursday, the last day of the party's convention in Philadelphia.

Her emails were the subject of a Federal Bureau of Investigation probe that found no basis for criminal charges despite what FBI Director James Comey this month called evidence that Clinton was "extremely careless" in her handling of classified information.

In the same news conference on Wednesday, Trump dismissed suggestions that Russia had sought to influence the US election by engineering the theft of embarrassing Democratic Party emails released by WikiLeaks last week.

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 theory. He suggested that China or some other party could be involved.

US President Barack Obama said it was possible that Russia would try to influence the US presidential election, after a leak of Democratic National Committee emails that experts have attributed to Russian hackers.

"Anything is possible," Obama told NBC News in an interview broadcast on Tuesday when asked if the Russians would try to influence the Nov. 8 election.

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.

Newt Gingrich, a former speaker of the US House of Representatives and a Trump supporter, said on Twitter that Trump was joking when he suggested Russia hack Clinton. But Trump repeated the call on Twitter, saying if anyone had Clinton's emails, "perhaps they should share them with the FBI!"

The Clinton campaign was not amused. "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. Trump's running mate, Indiana Governor Mike Pence, said 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," Pence said in a statement.

During the pugilistic news conference at his Doral golf resort, which lasted more than 45 minutes, Trump also said Putin would respect him if he were elected and called current President Barack Obama, a Democrat, the most "ignorant" president ever.

Donald Trump, the US Republican presidential nominee, Wednesday tried to portray just-concluded the Democratic convention while disputing critiques of his own campaign,  at a news conference in which he attacked President Barack Obama as historically "ignorant" and said a Hillary Clinton administration would be more of the same.

Trump protested a lack of mentions about Islamic State terrorists, little talk about police officers getting killed, and a paucity of American flags at the Democratic convention in Philadelphia; he criticized Mrs. Clinton for a "lack of loyalty" to ousted party Chairwoman Debbie Wasserman Schultz and claimed the Democrats offered no new ideas to lead the country.

"There's no change," Trump said in a news conference from his golf club in Miami, Florida. "It's going to be an extension of Obama."

Later, just hours before Obama was to address the Democratic convention, Trump bashed him as "the most ignorant president in our history," and added that "I believe that Hillary Clinton will be even worse."

Trump also protested what he called unfair media coverage and chided Clinton for not holding news conferences of her own.

As for attacks on him, Trump mocked Democratic complaints that Russia is behind the hack of Democratic National Committee emails that led to Wasserman Schultz's removal, all in an effort to help Trump politically. "It's just a total deflection, this whole thing with Russia," Trump said.

Trump said he knows nothing about the hack and suggested that Russia may not be responsible ("Nobody knows who did it"). He added that whoever did the hack probably has emails that Clinton "lost" or deleted from the private account she maintained while at the State Department, the subject of a recent FBI investigation.

Reuters/Special Correspondent