Top US nuclear war researcher Greg Mello said the potential for conflict between the two military superpowers is rising.
Relations have turned sour in recent weeks after Donald Trump bombed a Syrian airbase in revenge for a chemical attack.
Moscow has warned of “grave consequences” for “global security” if Trump signs off more military action in Syria.
Mr Mello said another “serious” US attack on Syria would “tilt the balance towards World War 3” and threaten a nuclear apocalypse.
The executive director of Los Alamos Study Group told Global Research that nuclear war would cause the collapse of systems that “keep everybody alive”.
He said: “To a first approximation, in a nuclear war between the US and Russia, everybody in the world would die.
“Some people in the southern hemisphere might survive, but probably not even them.
“Even a couple of nuclear weapons could end the United States as a government and an economy.
“It wouldn’t take a great deal to destroy the ‘just in time’ supply chains, the financial markets and the internet.
“The whole system is very fragile, especially with respect to nuclear weapons.”
US secretary of state Rex Tillerson said relations with Russia are “at a low point” after a meeting in Moscow last week.
He said: "The world's two foremost nuclear powers cannot have this kind of relationship."
Following a tense summit, Daily Star Online revealed the US targets Vladimir Putin will strike in World War Three.
Both nations offically adopt a policy of mutually assured destruction – meaning both sides would be completely annihilated in the event of a nuke war.
But Mr Mello said even a “limited nuclear war” would wipe out large swathes of the US.
He added: “Keep in mind that nuclear war is not one or two Hiroshima-sized bombs.
“The imagination cannot encompass nuclear war. Nuclear war means nuclear winter.
“It means the collapse of very fragile electronic, financial, governmental, administrative systems that keep everyone alive.
“We’d be lucky to reboot in the early 19th century. And if enough weapons are detonated, the collapse of the Earth’s ozone layer would mean that every form of life that has eyes could be blinded.
“The combined effects of a US-Russian nuclear war would mean that pretty much every terrestrial mammal, and many plants, would become extinct.
“There would be a dramatic biological thinning.”
His comments come as Mike Pence continued sabre rattling over North Korean nuclear threats during a visit to Japan.
He warned of an “overwhelming and effective response” if Kim Jong-un launches more missiles.
