China has expressed serious concern to Washington and Seoul after the US military started moving parts of its controversial THAAD anti-missile defence system to a deployment site in South Korea.

Key points: A spokesman said China thinks the deployment will 'worsen regional tensions'

A spokesman said China thinks the deployment will 'worsen regional tensions' China is concerned the system's radar can penetrate its territory

China is concerned the system's radar can penetrate its territory China is crucial to US-led efforts aimed at reining in North Korea

The issue also triggered protests from villagers in South Korea, amid tension over weapons development in North Korea, which days ago conducted a large live-fire exercise to mark the foundation of its military.

Speaking at a daily news briefing in Beijing, Foreign Ministry spokesman Geng Shuang said China urged the US and South Korea to withdraw the system.

Seoul and Washington say the sole purpose of the Terminal High Altitude Area Defense (THAAD) system is to defend against North Korean missiles, but China is concerned the system's powerful radar could penetrate its territory and undermine its security, and has repeatedly expressed opposition to it.

North Korea conducted a large live-fire exercise to mark its military's foundation. ( Reuters: Korean Central News Agency )

"China strongly urges the United States and South Korea to stop actions that worsen regional tensions and harm China's strategic security interests and cancel the deployment of the THAAD system and withdraw the equipment," Chinese foreign ministry spokesman Geng Shuang told a briefing.

"China will resolutely take necessary steps to defend its interests," Mr Geng said, without elaborating.

China is North Korea's sole major ally and is seen as crucial to US-led efforts to rein in its bellicose, isolated neighbour.

The United States began moving the first elements of the system to South Korea in March. ( Reuters )

The United States began moving the first elements of the system to South Korea in March after the North tested four ballistic missiles.

South Korea's defence ministry said elements of THAAD were moved to the deployment site, on what had been a golf course, about 250 kilometres south of the capital, Seoul.

"South Korea and the United States have been working to secure an early operational capability of the THAAD system in response to North Korea's advancing nuclear and missile threat," the ministry said in a statement.

The battery was expected to be operational by the end of the year, it said.

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The earlier-than-expected steps to deploy the system was also denounced by the frontrunner in South Korea's presidential election on May 9.

Liberal politician Moon Jae-in called for a delay in the deployment, saying the new administration should make a decision after gathering public opinion and more talks with Washington.

A spokesman for Mr Moon said moving the parts to the site "ignored public opinion and due process" and demanded it be suspended.

Television footage showed military trailers carrying equipment, including what appeared to be launch canisters, to the battery site

Protesters shouted and hurled water bottles at the vehicles over lines of police holding them back.

The Pentagon said the system was critical to defend South Korea and its allies against North Korean missiles and deployment would be completed "as soon as feasible".

A US military vehicle amid South Korean police officers trying to block protesters who oppose a plan to use the THAAD system. ( AP: Kim Jun-hum )

Reuters