SEOUL, South Korea — Tensions between South Korea and China over how to deal with the North have flared into an unusually blunt diplomatic dispute, with Seoul telling Beijing on Wednesday not to meddle in its talks with the United States over the possible deployment of an American missile-defense system here.

Jung Youn-kuk, a spokesman for President Park Geun-hye of South Korea, said Seoul’s decision to discuss the system, known as Thaad, which stands for Terminal High Altitude Area Defense, was based on its own need for “self-defense against North Korea’s growing nuclear and missile threats.”

“This is a matter we will decide upon according to our own security and national interests,” Mr. Jung said Wednesday. “The Chinese had better recognize this point.”

A senior official, speaking to reporters at the South Korean Foreign Ministry on the condition of anonymity, went further, advising China to “look into the root of the problem if it really wants to raise an issue with it” — a reference to the North’s pursuit of nuclear and ballistic missile technology and what South Koreans and Americans consider China’s failure to dissuade Pyongyang from that path.