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 advisor Jake Sullivan.

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