China has warned it will take "necessary measures" after the US accelerated plans to set up a missile defence system in South Korea.

American missile launchers and other equipment needed to build the Terminal High-Altitude Area Defense (THAAD) system arrived in South Korea on Tuesday, a day after North Korea continued its recent tests by firing four ballistic missiles into the ocean near Japan.

The THAAD system, which is due to be housed on a former golf course by the end of the year, can intercept and destroy short and medium range ballistic missiles during the last part of their flights.

South Korean media has speculated the THAAD deployment could be completed as early as April. Credit: AP

Washington and Seoul say the system is defensive and not meant to be a security threat to China or Russia, who have both joined Pyongyang in condemning the radar project.

But the Chinese Foreign Ministry said the US and South Korea would bear the "consequences" as it issued an angry condemnation.

China firmly opposes the deployment of THAAD. We will definitely be taking necessary measures to safeguard our own security interest. Chinese Foreign Ministry spokesperson

Adm. Harry Harris, head of the US Pacific Command, defended the action as the Osan air base in Pyeongtaek took the latest delivery.

He said the "continued provocative actions" by North Korea, including Monday's launch, "only confirm" the decision to build THAAD across the border.

Protesters opposed to the THAAD project have rallied in front of the defence ministry in Seoul, South Korea. Credit: AP

South Korean media has speculated the THAAD deployment could be completed as early as April on the golf course site provided by the South Korean retail giant Lotte.

Chinese protests have been held against Lotte while the South Korean government has raised concerns about a reported ban on Chinese tour groups visiting the country.