"With a large war between the United States and Russia, we are talking about the possible -- not certain, but possible -- extinction of the human race,” Helfand, the co-president of International Physicians for the Prevention of Nuclear War, said in a report released on Tuesday.
"In this kind of war, biologically there are going to be people surviving somewhere on the planet but the chaos that would result from this will dwarf anything we've ever seen," he added.
There are currently more than 17,000 nuclear warheads which have been ignored since the Cold War ended in 1991, according to the study called “Nuclear Famine: Two Billion People at Risk?"
Helfand said modern nuclear weapons are far more powerful than the atomic bombs the US used against Japan in 1945 which killed more than 200,000 people.
The scientist warned that even a limited nuclear war-- involving just less than 0.5 percent of the world's nuclear arsenal-- would put 2 billion lives at grave risk with more than 20 million people losing their lives within the first week of the explosions.
The ecological consequences of a nuclear war would put the survival of the entire planet at risk, according to the study. The firestorms caused by such a war would send approximately 5 million tons of soot into the atmosphere, blocking out sunlight and dropping temperatures across the planet.
Helfand painted a scenario where this climate hazard would cause “a nuclear famine” across the world. Hundreds of millions of people would face severe food shortages for years.
Nine countries are believed to possess nuclear weapons, with Russia and the United States holding the vast majority of them. Israel is the Middle East's sole possessor of nuclear weapons.
In 2009, Washington pledged to work toward the abolition of its nuclear weapons, but said it would keep them so long as others keep theirs.
In May 2012, seventeen nations issued a joint statement called for the total elimination of nuclear weapons. By this autumn the number rose to 125 nations