Geneva (AFP) - Pentagon Papers whistleblower Daniel Ellsberg called Friday on the Pentagon chief to "protect us" from President Donald Trump by resisting orders to launch military actions or use nuclear weapons.

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

He spoke shortly before the news broke on Saturday that the United States, Britain and France -- three nuclear powers -- were carrying out a wave of punitive strikes against Bashar al-Assad's regime in Syria over an alleged chemical weapons attack.

"It is a very dangerous time," Ellsberg 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.

International actions in Syria, involving four nuclear-armed states, 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.

- Resist 'reckless' orders -

Ellsberg famously leaked thousands of documents nearly half a century ago revealing that successive US administrations had lied to the public about the Vietnam war.

He insisted that now Defense 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 Defense 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.

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

Ellsberg said there was a desperate need for high-level people to be willing "to put their careers on the line" to do the right thing and help avert catastrophe.

"I think Mattis in particular, and (top US general Joe) Dunford, are cooling the president down from his threats of a catastrophic action in North Korea and even in Syria," he said.

He insisted that without a declaration of war by Congress or a UN Security Council directive, any order to go to war "would be a criminal order and they should not obey it".

But, he cautioned, "that doesn't mean that they will do what they should and resist him or disobey him if he insisted over time."

"They are slowing him down and that gives a chance for Congress to take action, but Congress seems to show no willingness to do that... So the situation is very dire at this point," he said.

- 'The Doomsday Machine' -

Ellsberg is a staunch anti-nuclear weapons campaigner who recently published a massive tome about the nuclear threat seen from the inside, titled "The Doomsday Machine: Confessions of a Nuclear War Planner".

He told AFP he was currently in Washington lobbying members of Congress about the dangers of reckless war plans and ever more powerful nuclear arsenals on hair trigger alerts.

He said he hoped to drum up support for a study on the consequences of nuclear war in terms of "nuclear winter" -- a situation where smoke blocks out the sun, dooming harvests and leaving basically anyone not killed in the blast or by radiation to starve to death.

He said his comments had been met with a lot of interest.

"I think people really are worried about Trump being the person who has his finger on what he calls his very big nuclear button. It doesn't reassure anybody," he said.

Ellsberg acknowledged that Trump's threatening and volatile rhetoric towards nuclear armed North Korea, which has drawn comparisons with Nixon's "madman theory" of diplomacy, could potentially scare adversaries into backing away from conflict.

"It can work, but it does that at the price of the risk of maintaining a doomsday machine in the world which is capable of reacting to a false alarm and ending most human life on earth," he said.

"That price is absolutely unconscionable and outrageous."

One of the world's most famous whistleblowers, Ellsberg said there was currently a desperate need for more people willing to reveal the true dangers lurking behind the curtain.

He admitted that the risks to whistleblowers "are very high".

"But the chances I would say of averting a war on North Korea or in Syria, Iran or elsewhere, would be well worth the personal costs of life in prison or even death," he said.

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