The allies had been set to sign an agreement today on setting up a joint working group to look into the roll-out of the Terminal High Altitude Area Defence System (THAAD) against North Korea's growing missile threat.

"The related accord is in the final stages but has been postponed by a day or two because of last-minute negotiations," ministry spokesman Moon Sang-Gyun said.

The THAAD system fires anti-ballistic missiles into the sky to smash into enemy missiles either inside or outside the Earth's atmosphere during their final flight phase.

The interceptor missiles carry no warheads, instead relying on kinetic energy to destroy their targets.

More than two weeks ago, the allies announced their intention to begin talks on its deployment following Pyongyang's long-range ballistic missile launch on February 7 but negotiations to launch the Joint Working Group were protracted.

The delay comes as Chinese Foreign Minister Wang Yi is set to visit Washington from today to meet his US counterpart John Kerry for possible talks over the controversial defence system and North Korea.

China opposes the proposed deployment of THAAD, with foreign ministry spokeswoman Hua Chunying warning Monday that it should not be used as a front to "undermine China's own legitimate (security) interests".

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