SEOUL—North Korea is completing a major expansion of a key missile-manufacturing plant, said researchers who have examined new satellite imagery of the site, the latest sign Pyongyang is pushing ahead with weapons programs even as the U.S. pressures it to abandon them.

The facility makes solid-fuel ballistic missiles—which would be able to strike U.S. military installations in Asia with a nuclear weapon with little warning—as well as re-entry vehicles for warheads that Pyongyang might use on longer-range missiles able to...