North Korea appears to have secured all core mid- and long-range missile technologies except for atmospheric re-entry technology, South Korea's military officials said Tuesday, noting the communist state has made strides in improving delivery vehicles through repeated tests.

Pyongyang has acquired key technologies concerning the propulsion system, guidance and control system, rocket separation and other crucial elements through a total of six long-range rocket tests including the latest in February, the officials said.

To carry out regular tests of rockets, the North has also constructed an automated launch site in Dongchang-ri in the country's northwest, indicating Pyongyang has made considerable efforts to develop a nuclear-tipped intercontinental ballistic missile.

The North's test of the three-stage rocket, conducted last month, has been considered successful as it put what it claims is a satellite into orbit for the second consecutive time following its previous test in December 2012.  

The liquid-fuel propulsion system of the first-stage rocket was made with four Rodong missile engines, while the second-stage rocket carried a single Scud missile engine, the officials said, indicating that the technology to combine each stage of the rocket has become "stable."

"It is not wrong to say that the North has entered some mature technological phase in terms of its technology to produce multiple-stage rockets and technology for the separation of each stage during flight," a military official said, declining to be named.

In a 2012 test, the North also affirmed that it had reliable technology to produce a three-stage rocket based on Scud and Rodong missile technologies. Scud missiles have ranges of 300-500 km while Rodong missiles can fly around 1,300 km.

Pyongyang was also seen making improvements in terms of in-flight rocket altitude control. In a 2009 test, it attached a small thruster to a long-range rocket to ensure that the rocket soared in the right position. In a 2012 test, it also put a thruster on the propulsion system of the three-stage rocket to control the altitude.  

Despite much progress in missile technology, Pyongyang faces the toughest challenge of developing an operational ICBM: re-entry technology. As the intercontinental missile re-enters the earth at the speed of Mach 20, the rocket's warhead needs to be capable of withstanding temperatures of around 6,000-7,000 degrees Celsius.

On Tuesday, Pyongyang said it has acquired "reliable" re-entry technology. But Seoul officials dismissed the claim as the reclusive regime has yet to conduct any open test of its re-entry technology.

The North has deployed the Musudan missile with a range of more than 3,000 km. But it has yet to conduct any test on it. It has also been pushing to develop the KN-08 ICBM but without any confirmed test of re-entry capabilities. (Yonhap)