Republican Donald Trump has appealed to Russia to uncover and release 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 system.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump told reporters on Wednesday. Clinton has said the emails she did not hand over were private.

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," Trump said, suggesting that China or some other party could be involved.

Clinton, due on Thursday to accept the Democratic Party nomination to face Trump in the November 8 election, 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 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.
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.

Trump's running mate, Mike Pence, took a different approach from 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," Pence 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.