North Korea conducted its third nuclear test Tuesday, the latest step in a years-long effort to develop nuclear weapons. Experts believe the country remains far from having a nuclear-armed missile that could threaten the United States, which would require an accurate long-range rocket and a nuclear warhead that could be mounted on it. Here is a look at North Korea's progress so far:

1998:

— ROCKET LAUNCH (August): This early launch gets the world's attention, because it goes well beyond North Korea's known capability. The rocket, which hurtles over Japan, has an estimated potential range of 1,550 miles (2,500 kilometers), but accuracy is reportedly poor with no meaningful strike capability.

2006:

— ROCKET LAUNCH (July): A three-stage rocket with a potential range of 4,100 miles (6,700 kilometers) fizzles soon after liftoff, the U.S. and South Korea say. North Korea has never acknowledged the launch.

— NUCLEAR TEST (October): North Korea detonates a nuclear device for the first time, but the yield is a very low 0.5 to 1 kiloton.

2009:

— ROCKET LAUNCH (April): This launch is a partial success, with two of the three stages pushing the rocket out over the Pacific. The third stage fails, and, despite North Korea's claims of success, no satellite is put into orbit, the U.S. says. The rocket dubbed Unha-2 represents a significant advancement over previous rockets, according to experts. Continued...