The US military started moving parts of an anti-missile defence system to a deployment site in South Korea, triggering criticism from China amid tension over North Korea's weapons development.
South Korea's defence ministry said elements of Terminal High Altitude Area Defense (THAAD) were moved to the deployment site, on what had been a golf course, about 250km 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, and is adamant in its opposition.
"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," a Chinese foreign ministry spokesman told a briefing.
"China will resolutely take necessary steps to defend its interests," he said, without elaborating.
China is North Korea's sole major ally and is seen as crucial to US-led efforts to rein in its 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.
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.
ThePentagon 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".