The risk of millions of Americans dying from an accidental nuclear attack by a Russian submarine is rising, scientists report in Thursday's New England Journal of Medicine.

Citing what they called the steadily deteriorating condition of Russia's nuclear command system, the study's authors laid out a scenario of a plausible accidental attack: a single Russian nuclear submarine launching its weapons against the United States.Nearly 7 million people would be killed instantly from the thermonuclear explosions in the eight cities researchers chose arbitrarily.

They selected the submarine scenario because "the Russian general staff cannot continuously monitor the status of the crew and missiles."

As the study's co-author Theodore Postol, now of the Massachusetts Institute of Technology and a former Navy weapons expert, told a Boston news conference: the Russians "have had a lot of trouble with their submarine force historically."

Postol and the study's lead author, Dr. Lachlan Forrow of Beth Israel Deaconess Medical Center, said authorities believe that a launch based on a false warning of a nuclear attack would be the most plausible scenario for an accidental attack.

They noted that several false alarms had already occurred in both the United States and Russia. Each country maintains their nuclear arsenals on high alert, the researchers said.

In prose similar to a thriller's, the scenario details an attack by a single Russian nuclear submarine, armed with 16 missiles carrying 48 100-kiloton nuclear warheads, launched against the continental United States from the Barents Sea. The missiles have a range of 5,150 miles.

Researchers estimated 6.83 million people in Atlanta, Boston, Chicago, New York, Pittsburgh, San Francisco, Seattle and Washington, D.C., would be instantly killed by the thermonuclear blasts.

"An attack such as the one we have described would not only cause more than 6 million immediate Americans' deaths in nuclear firestorms we have conservatively estimated, and not only cause the hundreds of thousands or millions of additional casualties as a result of radiation injuries, but plausibly trigger a U.S. nuclear response," Forrow said.

"During a period when Russia is not even our adversary, this is politically, medically and morally obscene," he said.