WASHINGTON—With concerns rising about a possible North Korean long-range missile test this weekend, two independent scientists say the regime may be using an old Soviet ballistic missile to boost a rocket capable of reaching the West Coast of the United States.

North Korea is not known to have nuclear warheads and faces years of research and testing before building such a reliable weapon.

But the scientists say that if North Korea does have such a Russian-made ballistic missile in its arsenal, it could modify the rocket into a two-stage missile that could reach Seattle, Wash., carrying a 900-kilogram warhead, or San Francisco carrying a 700-kilogram charge.

The design of a long-range missile tested by North Korea last April “represents a very significant advance in rocket technology,” said Massachusetts Institute of Technology professor Ted Postol and Union of Concerned Scientists’ David Wright in a June 29 assessment published in the Bulletin of Atomic Scientists.

Using data and imagery from North Korea’s April 4 launch, Postol and Wright calculated that the second stage of the North Korean rocket had the external dimensions, engine power and key features of an SS-N 6, a Soviet submarine-launched ballistic missile first deployed in 1968.

Their theory is at odds with U.S. officials’ skepticism of the recent North Korean long-range missile launch, dismissed as a failure.

Missile expert and former U.N. arms inspector Mike Elleman cautioned against assuming that the similarities between the external dimensions of the North Korean second stage and the SS-N 6 mean that the two are the same technology.

But Elleman added that the coincidence is hard to explain.

Geoffrey Forden, another missile expert with MIT, sees merit in the Russian missile theory and believes North Korea may have its own production line for SS-N 6 missile components.