The comments by US Assistant Secretary of State Daniel Russel, who is visiting Seoul, came after China's Assistant Minister of Foreign Affairs Liu Jianchao expressed concern yesterday over the idea of a US missile defence system on South Korean soil.
Liu's statement followed similar comments made by Chinese Defence Minister Chang Wanquan during his visit to Seoul last month. China sees the system as designed to curb its own military might.
US military authorities say they have conducted "informal studies" to find suitable sites for a possible future deployment of a Terminal High-Altitude Area Defense (THAAD) battery. But they have stressed that no decisions have been made to deploy such a system.
Russel said South Korea and the United States face a "signficant threat from North Korea's growing ballistic missile programme", which he said the North pursues in violation of international laws.
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"How they do it when they do it is something that the experts will have to determine but I think that it is for the Republic of Korea to decide what measures it will take in its own alliance defence", he added.
Walking a diplomatic tight rope between China and the United States, South Korea remains ambivalent on the sensitive security issue, saying there have been neither consultations with the United States nor a decision on the matter.