South Korea's defence chief and his new US counterpart vowed Tuesday to push ahead with a plan to deploy a US anti-missile system this year, Seoul's military said, despite angry protests by China.

The two allies last year announced the plan to deploy the Terminal High Altitude Area Defense (THAAD) system following a series of atomic and missile tests by nuclear-armed North Korea.

New US Defense Secretary James Mattis and his South Korean counterpart Han Min-Koo reaffirmed the plan in a phone conversation, Seoul's defence ministry said, days before Mattis visits Asia.

How the incoming administration of President Donald Trump will approach the region has raised worries in South Korea and Japan, both of them longstanding US allies.

During his campaign, Trump threatened to withdraw US forces from the two countries if they did not step up their financial support for their defence.

Mattis and Han voiced grave concern over growing threats from North Korea and agreed on "a need...to push ahead with the deployment of the THAAD as planned", the ministry said in a statement.

The two ministers also warned that the North may stage armed provocations at times of power transitions in the South and the US.

Their conversation came after Trump and South Korea's acting president Hwang Kyo-Ahn agreed Monday to strengthen their joint defence capabilities, but with the White House's official statement making no specific reference to THAAD.

The North's leader Kim Jong-Un boasted early this month that the country was in the "final stages" of developing an intercontinental ballistic missile.

The THAAD plan has infuriated China, which fears it will undermine its own ballistic capabilities and has slapped a series of measures seen by Seoul as economic retaliation.

It also sparked angry protests from residents of the area chosen to host the powerful missile system, with several South Korean presidential hopefuls vowing to scrap the plan if elected.

Mattis, during his first overseas tour since taking office this month, visits South Korea from Thursday to Friday before flying to Japan.

He is expected to meet with Han as well as Hwang, who has been serving as acting head of state since parliament impeached President Park Geun-Hye last month.

The South's Constitutional Court is reviewing the validity of the motion. If the court confirms the impeachment, a presidential election will be held within 60 days.