The new site is thought to be more sophisticated than the Musudan-ri site
North Korea is close to completing a second launch site for long-range missiles, reports say.
The existence of the site, said to be 30 miles (50km) from the Chinese border, was made public by an analyst using satellite imagery.
Reports say South Korea's defence minister Lee Sang-Hee told a closed-door parliamentary session that the project was about 80% completed.
"We're watching it closely," he was quoted as saying by AFP news agency.
The site's construction does not breach any international agreement.
A US official told CNN that the intelligence community had known about the secret facility for several years and was monitoring it closely.
Joseph Bermudez, a senior analyst at Jane's Intelligence Group, located the Tongch'ang-dong site earlier this year with the help of a private satellite imagery analysis company, Talent-keyhole.com.
It is built on the site of a small village called Pongdong-ni which, he said, was displaced during construction.
It has not yet been used and could take another one to two years to complete, Mr Bermudez said.
However, it is larger, more costly and more sophisticated than the Musudan-ri launch site on North Korea's east coast, Mr Bermudez was quoted by CNN as saying.
"This is a national programme in their desire to expand both their space launch and ballistic missile launch programmes," he said.
BBC diplomatic correspondent Jonathan Marcus says the location in itself does not significantly alter the range of targets that North Korea could potentially strike.
But the existence of the base is a clear sign that North Korea's missile programme is active.
Western experts believe that Pyongyang's ultimate goal is to have an inter-continental range system that could potentially deliver a nuclear pay-load, our correspondent adds.
North Korea used Musudan-ri to launch a Taepodong-1 missile in 1998 over Japan. A Taepodong-2 missile was launched from there in 2006, but it failed.
News of the missile site comes as international efforts to urge North Korea to abandon its nuclear programme are at an impasse.
North Korea agreed in February 2007 to give up its nuclear ambitions in return for aid and diplomatic concessions, but the progress of the deal has been far from smooth.
After a long delay, Pyongyang handed over details of its nuclear facilities in June 2008. In return, it expected the US to remove it from the list of state sponsors of terrorism.
But the two sides cannot agree on a process to verify the information that North Korea handed over and Pyongyang now appears to be starting to reassemble its main nuclear plant.