What you need to know about the missile defense system US is setting up in S. Korea

PHOTO: U.S. military vehicle moves past banners opposing a plan to deploy an advanced U.S. missile defense system called Terminal High-Altitude Area Defense, as South Korean police officers stand guard in Seongju, South Korea, on April 26, 2017. PlayKim Jun-hum/Yonhap via AP Photo
WATCH US setting up missile defense system in South Korea

The United States has begun moving parts of an antimissile system into their deployment site in South Korea to help protect that nation from a possible North Korean missile attack.

The system has been long-planned but the overnight deployment was ahead of the expected schedule, surprising some South Koreans and sparking protests by hundreds of residents.

Parts of the antimissile defense systems were moved to a former golf course in the southern area of the country, about 135 miles southeast of Seoul. The system will be operational by the end of the year, according to the South Korean defense ministry.

Here's what you need to know about the latest developments in the region.

What is the THAAD system?

The Terminal High Altitude Area Defense (THAAD) system is designed to intercept incoming short- and medium-range missiles. The United States and South Korea announced in July 2016 that the system would be deployed to South Korea after a series of North Korean missile launches last year.

The first elements of the THAAD system arrived in the country last month, the day after North Korea fired four medium-range missiles into the Sea of Japan, some of them traveling as far as 600 miles. A U.S. defense official said the system's arrival in South Korea was coincidental and had been long-planned.

"The timely deployment of the THAAD system by U.S. Pacific Command and the secretary of defense gives my command great confidence in the support we will receive when we ask for reinforcement or advanced capabilities," Gen. Vincent K. Brooks, the commander of U.S. Forces Korea, said when the elements arrived.

Wednesday's overnight deployment placed parts of the system that were already in the country into their permanent position on the golf course.

The deployment of the missile system has experienced many delays since last summer's agreement between the United States and South Korea. It was only last month that South Korea announced the missile system would be located on the golf course it had acquired.

The United States also maintains 28,500 troops in South Korea to help deter any North Korean aggression.

Chinese opposition to THAAD

The THAAD system's deployment has been opposed by China, which has claimed it could contain its own missile systems and security interests in the region. U.S. defense officials have countered that the system is strictly defensive in nature and intended solely for South Korea's protection.

At a briefing last month, Chinese Foreign Ministry spokesman Geng Shuang said, "China firmly opposes the deployment of THAAD. We will definitely be taking necessary measures to safeguard our own security interest. All consequences entailed from that will be borne by the U.S. and (South Korea). We once again strongly urge the relevant sides to stop the process of deployment and refrain from going further down that wrong path."

He today again urged the U.S. and South Korean governments to halt THAAD's deployment, saying it disrupts "regional strategic equilibrium" and further aggravates the tense situation on the Korean Peninsula, according to The Associated Press.

ABC News' Elizabeth McLaughlin contributed to this report.