The US military on Wednesday started moving parts of its THAAD anti-missile defence system to a site in South Korea, triggering protests from villagers and criticism from China.
The steps to deploy the Terminal High Altitude Area Defense (THAAD) system, which came earlier than expected, were also denounced by Moon Jae-in, the frontrunner in South Korea's presidential election on 9 May.
South Korea's defence ministry said elements of THAAD were moved to the deployment site on what had been a golf course about 250 km 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.
The United States and South Korea agreed last year to deploy the THAAD to counter the threat of missile launches by North Korea. They say it is solely aimed at defending against North Korea.
But China says the system's advanced radar can penetrate deep into its territory and undermine its security, while it will do little to deter the North.
"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 in Beijing.
"China will resolutely take necessary steps to defend its interests," 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 neighbour.
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 has accused China of discriminating against some South Korean companies operating in China because of the deployment.
Moon Jae-in, the liberal politician expected to win South Korea's election, has 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 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".
More than 10 protesters were injured, some of them with fractures, in clashes with police, Kim Jong-kyung, a leader of villagers opposing the deployment, told Reuters.
Kim said about 200 protesters rallied overnight and they would keep up their opposition.