PHILADELPHIA: Hillary Clinton's campaign on Wednesday lashed out at Donald Trump for encouraging Russia to trawl the former secretary of state's emails, describing his comments as a “national security issue.”

“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,” said top Clinton adviser Jake Sullivan.

“This has gone from being a matter of curiosity, and a matter of politics, to being a national security issue.”

Trump, who is expected soon to receive his first government intelligence briefing, suggested on Wednesday that Russia could help find emails known to have been deleted from Clinton's private server when she was secretary of state on the grounds they were personal.

The FBI concluded earlier this month that Clinton had been “extremely careless” in her handling of classified material via a private email server, but did not recommend that she face criminal charges. Republicans see the missing emails as a smoking gun, however.

“Russia, if you're listening, I hope you're able to find the 30,000 e-mails that are missing,” Trump told a news conference.

The Clinton campaign's explosive accusation of inviting foreign spying came as US intelligence agencies pointed to Russia as the cause of a mass hack of Democratic Party emails.

Clinton's camp believes that Moscow gave the mails to WikiLeaks, which released them last week, to foment unease between the former first lady and her one-time Democratic rival Bernie Sanders.

The scandal caused the resignation of Democratic Party leader Debbie Wasserman Schultz and poured kerosene on Democratic infighting at a party convention in Philadelphia.

Trump has adopted a number of pro-Russian policy positions as a presidential candidate, suggesting he would recognize Moscow's annexation of Crimea and lift economic sanctions.

His campaign chair's close ties with pro-Russian politicians in Ukraine has also raised allegations that Trump is too cozy with Moscow and its powerful leader Vladimir Putin.

“I have nothing to do with Putin,” Trump said. “Never spoken to him. I know nothing about him other than he will respect me.”

“If it is Russia. Nobody knows. It's probably China, or it could be somebody sitting in his bed. But it shows how weak we are. It shows how disrespected we are,” he said.

Trump's vice-presidential running mate Mike Pence quickly tried to limit the fallout from the mogul's comments.

“The FBI will get to the bottom of who is behind the hacking,” he said in a statement.

“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.”