What you need to know about the US missile defense system now active 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 antimissile system recently deployed to South Korea to help protect that nation from a possible missile attack from North Korea is now operational, a U.S. official confirms to ABC News.

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Though the system has been long-planned, the United States began moving parts of an antimissile system into its deployment site ahead of schedule last week, surprising some South Koreans and sparking protests by hundreds of residents.

While the system is now capable of intercepting a North Korean missile, according to U.S. Forces Korea, it will not be fully operational for a few months.

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, consisting of multiple missile batteries coordinated by a radar and tracking system.

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 in March, the day after North Korea fired four medium-range missiles into the Sea of Japan, some 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.

Last week's overnight deployment placed parts of the system that were already in the country into their permanent position on the golf course, about 135 miles southeast of Seoul.

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 in March, 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."

Last week, he 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.

Paying for THAAD

President Trump mentioned in an April interview that it would be appropriate for South Korea to pay for the system's deployment.

"It's a billion dollar system," Trump told Reuters.

Lt. Gen. H.R. McMaster, the national security adviser, told Fox News on Sunday that the United States would adhere to the agreed upon terms of the missile's deployment.

"But what the president has asked us to do is to look across all of our alliances and to have appropriate burden-sharing, responsibility-sharing," said McMaster. "We are looking at that with a great ally, South Korea."

He added, "The question of what is the relationship on THAAD, on our defense relationship going forward, will be renegotiated as it's going to be with all of our allies."

ABC News' Elizabeth McLaughlin contributed to this report.