Russian military analyst Vladimir Yevseyev of the Moscow-based CIS Institute also said the designs of North Korean submarine-launched ballistic missiles are based on Russian models from the '90s, South Korean news agency Yonhap reported.
Pyongyang has yet to acquire all of the skills needed to make indigenous sub-launched missiles that are not Russian copies, Yevseyev said.
North Korea's missiles keep exploding in mid-air because of "abnormal combustion in the engine fuel" of the Musudan, the analyst said.
But the cause of the combustion is not known, Yevseyev added, saying it could be related to a new kind of fuel being used in the rockets, rather than the liquid fuel that was used in the existing Rodong missiles.
"There could be a problem with the fuel or the oxidizer. Or, there could be a problem with the design," Yevseyev said.
Although liquid-fuel missiles like the Musudan are more stable compared to solid-fuel missiles, if the Musudan missile continues to fail to launch at the initial stage, that means the missile system is "unreliable," the analyst said.
North Korea began testing the Musudan in 2007 and Pyongyang has claimed that 50 of the missiles have been placed on standby. But Yevseyev said there are only about eight launchers, including two road mobile launchers that can be used to shoot the rockets.
Any rocket launch would be a violation of U.N. Security Council resolutions, and the international community has repeatedly condemned Pyongyang for its acts of provocation.
Yevseyev served as a Russian military commander specializing in intercontinental ballistic missiles until 2001.