The US and South Korea have been discussing the US Terminal High Altitude Area Defense (THAAD) system as a solution. But Chinese opposition to a South Korean Anti-Ballistic Missile (ABM) (or ballistic missile defence (BMD)) system on its doorstep may sway the US Congress or hesitant Obama away from approving THAAD for South Korea. Possible US blockage or hesitation on approving THAAD may be encourage South Korea to make its existing medium-tier KM-SAM system an "upper tier" system.
South Korea working with Russia on such a sensitive strategic missile program surprised me but its true. South Korea appears to be hedging in its alliance building by nurturing substantial links to China and Russia while maintaining the traditional US alliance.
The KM-SAM (Cheolmae-2) is currently the middle-tier of South Korea's three-tier aerial and missile defence program. The middle-tier KM-SAM was due to enter service by late 2015, replacing the aging MIM-23 Hawk missile system. Current KM-SAM performance may be the ability to intercept targets up to a ceiling of 15 km at a range of 40 km.
It appears that it would take South Korea much longer to develop an upper-tier KM-SAM than receive THAAD from the US (if the US doesn't hesitate).
The KM-SAM (above called "MR-SAM"). It has a cold launch (using compressed gas) then the missile's rocket motor ignites and the missile is guided by radar. The missile can change direction quickly and with a low IR signature has little chance of being detected by the enemy. In cooperation with Russia, a Korean engineering team replaced the existing large Russian radar system with a smaller truck mounted radar.