WASHINGTON (Reuters) - Deployment of a new U.S. missile-defense system to South Korea "is going to happen," U.S. Defense Secretary Ash Carter said on Friday, adding that China should do more to counter North Korea's missile development rather than complain about U.S. plans.

The United States and South Korea began talks on possible deployment of the Terminal High Altitude Area Defense (THAAD) system after North Korea tested its fourth nuclear bomb on Jan. 6 and launched a long-range rocket on Feb. 7.

China agreed to tough new U.N. sanctions on North Korea after the tests but has said it is "firmly opposed" to THAAD deployment, arguing that it will undermine its strategic deterrent.

Asked at a New York seminar if the deployment would go ahead, Carter replied:

"Oh, it’s going to happen. It’s a necessary thing. It’s between us and the South Koreans; it’s part of protecting our own forces on the Korean peninsula, and protecting South Korea.

"(It) Has nothing to do with the Chinese, and I do wish the Chinese would work with us, or really work bilaterally with North Korea more effectively, although it’s easy to say that - dealing with North Korea for anybody is a challenge - at heading off their missile challenge in the first place.

"But we need to defend our own people, we need to defend our own allies, and we are going to do that," Carter told the Council on Foreign Relations.

China complains that the THAAD has a range that would extend far beyond the Korean peninsula and into China and deployment of the system would threaten its national security interests and damage regional strategic stability.

U.S. officials believe that China, as North Korea's neighbor and only major ally, is best placed to influence the isolated country, although some analysts believe this ability to push for change has waned in recent years.

(Reporting by David Brunnstrom; Editing by Chizu Nomiyama)