Trump is Russia's 'unwitting agent' says ex-US spy chief

Presidential hopeful Donald Trump has been accused of being an "unwitting agent" of Russia by a former US spy chief.

Michael Morell is a 33-year veteran of the Central Intelligence Agency [CIA] and served as acting head of the spy agency twice.

Following a series of blunders by Trump during his campaign trail - including insulting the parents of a deceased army veteran - his Democratic rival Hillary Clinton has taken a comfortable lead in the race to the White House.

Trump's admiration for Russian President Vladimir Putin has also attracted his criticism, including from Morell.

"On November 8, I will vote for Hillary Clinton," he wrote in a New York Times column, "[Trump] may well pose a threat to our national security."

"Putin played upon Mr. Trump's vulnerabilities by complimenting him. He responded just as Mr. Putin had calculated," he added.

Trump has praised Putin as a great leader, and taken policy positions "consistent with Russian, not American, interests". 

This included endorsing Russian espionage against US figures like Clinton, Morell said.

"In the intelligence business, we would say that Mr. Putin had recruited Mr. Trump as an unwitting agent of the Russian Federation," he added.

Trump's running mate Mike Pence, the governor of Indiana, was dutifully dispatched to counter the assertion, telling NBC that "these people are playing politics".

Trump's election chief Paul Manafort has also been outed in a New York Times' for working with former pro-Moscow Ukrainian President Viktor Yanukovych.

Agencies contributed to this story.