"The situation is closer to the possible use of nuclear weapons since any time I would say in 50 years," Ellsberg, a former high-level military analyst and nuclear war planner turned world-famous whistleblower, told AFP in a telephone interview.

"It is very dangerous time," he said, pointing out that Trump's war of words with North Korea was the first time since the Cuban missile crisis in 1962 that a US president has threatened military action against another nuclear-armed state.

At the same time, there are currently four other nuclear armed states, Russia, Israel, Britain and France, in addition to the United States "running hostile actions" in Syria.

This, coupled with the political pressures on Trump at home, could easily spell disaster, Ellsberg warned.

"The president I'm afraid has a very strong temptation to start a war as cover for his firing the special prosecutor," he said, referring to Robert Mueller, who is investigating links between the Trump campaign and Russia in the 2016 election.

Ellsberg, who famously leaked thousands of documents nearly half a century ago revealing that successive US adminstrations had lied to the public about the Vietnam war, insisted that Defence Secretary Jim Mattis should be prepared to resist "impulsive and reckless" orders from Trump to go to war.

The 87-year-old pointed out that under President Richard Nixon, then defence secretary James Schlesinger had secretly insisted that no orders from the president to launch military action or use nuclear weapons should be obeyed unless it had his stamp of approval.

"Now that is an unconstitutional order, and it would be unconstitutional for James Mattis to make such a directive right now," Ellsberg said.

"Nevertheless, he ought to do it... We are depending on Maddog Mattis to protect us from a president."