WASHINGTON--Retaliatory nuclear attacks by North Korea against U.S. allies could kill 2.1 million people and injure 7.7 million in total in Tokyo and Seoul, according to an analysis by a Washington-based research institute.

The study, conducted by 38 North, a project of the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies, came up with the estimates based on the scenario of Pyongyang reacting to a U.S. strike against a nuclear or missile facility in North Korea or the interception of one of its ballistic missiles.

If the “unthinkable” happened and North Korea used nuclear weapons, the death toll would range from 200,000 to 940,000 in Tokyo and from 220,000 to 1.16 million in Seoul, according to the analysis.

If Pyongyang used thermonuclear weapons against the two densely populated capitals, up to 1.8 million would die in Tokyo and 2 million would be killed in Seoul, the analysis showed.

North Korea is believed to possess up to 25 ballistic missiles that are mounted with nuclear warheads. Each of these nuclear missiles could produce an explosion equivalent to 15 to 25 kilotons of TNT, according to 38 North.

Under the scenario, all of these weapons would be launched toward the two capitals.

The experts took into account the missile defense systems of Japan and South Korea as well as U.S. forces, and predicted three levels of probability for detonation of the 25 nuclear warheads: 20 percent, 50 percent and 80 percent.

The institute said the nuclear test conducted by North Korea on Sept. 3 was likely a thermonuclear device with estimated yields of between 108 and 250 kilotons of TNT, or up to 17 times the power of the atomic bomb dropped on Hiroshima in 1945.

The researchers warn that U.S. military action could significantly increase the risk of escalation by North Korea, leading to the use of nuclear weapons against Japan and South Korea.