WASHINGTON — With concerns rising about a possible North Korean long-range missile test this weekend, two independent scientists say the rogue 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 carrying a 900-kilogram warhead, or San Francisco carrying a 700-kilogram charge.