Hundreds of millions of more would die from disease and conflicts over food in the aftermath of any such war.
US medical expert Ira Helfand will today present this horrifying scenario in London during a conference at the Royal Society of Medicine.
"A limited nuclear war taking place far away poses a threat that should concern everyone on the planet," the New Scientist magazine quoted Helfand as saying.
"It is appropriate, given the data, to be frightened," said Helfand, who is an emergency-room doctor in Northampton, Massachusetts, US, and a co-founder of the US anti-nuclear group, Physicians for Social Responsibility.
Helfand has tried to map out the global consequences of India and Pakistan exploding 100 Hiroshima-sized nuclear warheads.
Referring to earlier studies that have suggested that in such a conflict, the annual growing season in the world's most important grain-producing areas would shrink by between 10 and 20 days, he said that the world is ill-prepared to cope with such a disaster.
"Global grain stocks stand at 49 days, lower than at any point in the past five decades," he said, adding: "These stocks would not provide any significant reserve in the event of a sharp decline in production. We would see hoarding on a global scale."
Countries, which import more than half of their grain, such as Malaysia, South Korea and Taiwan, would be particularly vulnerable, along with 150 million people in north Africa, which imports 45 percent of its food, Helfand said.
Many of the 800 million around the world who are already officially malnourished would also suffer, he added.
He went on to say that the global death toll from a nuclear war in Asia "could exceed one billion from starvation alone."
Food shortages could also trigger epidemics of cholera, typhus and other diseases, as well as armed conflicts, which together could kill "hundreds of millions".
Helfand further told the magazine that the smoke would warm the stratosphere by up to 50°C, accelerating the natural reactions that attack ozone.
"No-one has ever thought about this before...I think there is a potential for mass starvation," he cautioned.
Endorsing Helfand's views, John Pike, director of the US think tank, globalsecurity.org, said the fallout from a nuclear war between India and Pakistan "would be far more devastating for other countries than generally appreciated."
"Local events can have global consequences," he added.