Visiting US Defense Secretary James Mattis and Republic of Korea National Security Office chief Kim Kwan-jin met in Seoul on Thursday and agreed to press forward with deployment of the Terminal High Altitude Area Defense system in the ROK, a move that observers have said may destabilize Northeast Asia.

The Pentagon chief, who arrived in the ROK on Thursday, said the Trump administration will make a top security priority of nuclear threats from the Democratic People's Republic of Korea.

Mattis said before the visit that "no other nation" needs to be concerned about the THAAD system, Reuters reported.