Donald Trump has called on Russia to hack into Hillary Clinton's email, in an unprecedented request for a rival power to influence a presidential election though cyber attacks.

Mr Trump was discussing the recent breach of the Democratic National Committee, which President Barack Obama has said was almost certainly carried out by Russia and which caused damaging private emails to become public.

The Republican nominee said he did not know if Russia was behind that attack, but that he would like to see the Kremlin turn its attention to the 30,000 messages Mrs Clinton deleted prior to the FBI investigation into her email practices.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing. I think you will probably be rewarded mightily by our press," he said.

Mr Trump, who was giving a press conference in Florida, said it gave him "no pause" to essentially sanction Russian cyber hacking on an American official.

"Hey you know what gives me more pause? That a person in our government - Crooked Hillary Clinton - that a person in our government would delete or get rid of 30,000 emails," he said.

"Now, if Russia or China or any other country has those emails, I mean to be honest with you I'd love to see them."