A Pentagon assessment says any full-scale nuclear exchange between India and Pakistan could kill up to 12 million people immediately and injure up to seven million in the subcontinent.
The report says even a "more limited" nuclear war would have cataclysmic results, overwhelming hospitals across Asia and requiring vast foreign assistance, particularly from the United States, to battle radioactive contamination.
The New York Times quotes the report as saying more deaths could be caused by urban firestorms triggered by the nuclear exchange, or deaths from long-term radiation. Still more could die of hunger and disease.
American estimates of the number of warheads each has is classified. But the paper quotes Pentagon officials as saying in general terms that Pakistan has a couple of dozens, while India has several dozens.
The report comes amid mounting international pressure on the two neighbors to cool down their military tension.