South Korea's defense minister on Thursday ordered an early development and deployment of long-range ballistic missiles that can hit all parts of North Korea.

Minister Kim Kwan-jin visited a front-line unit to urge the military to stay vigilant against any provocations from the North after it threatened a third nuclear test in response to the U.N.

Security Council sanction for Dec. 12 rocket launch.

His inspection came after President Lee Myung-bak warned of "grave consequences" against the North in case of another atomic test in a meeting with top security ministers earlier in the day.

 "Many of North Korea's missile bases are located in the rear (northern region)," Kim told soldiers in Yeoncheon, north of Seoul.

"To be able to destroy the origin of provocations, ballistic missiles with an 800-kilometer range should be promptly put in place."

Concerns over a third nuclear test are growing, as recent satellite imagery show increased activity near a nuclear site in the North's northeastern tip, where the 2006 and 2009 tests were conducted.

After South Korea and the United States in October 2012 agreed to nearly triple Seoul's missile range to 800 kilometers, the military has worked on developing longer-range missiles. The missiles are capable of hitting all parts of North Korea.

The original plan calls for missile deployment by 2017, but the military is expected to advance the date by two years to 2015.

The latest move was seen as attempt to improve combat capability before Seoul regains its wartime operational control of its forces from Washington at the end of 2015.

South and North Korea share the world's most heavily fortified border. The two sides are still technically at war since the

1950-53 Korean War ended in a cease-fire instead of a peace treaty. (Yonhap News)