U.S., South Korea Agree on Deploying Advanced Antimissile System

Washington and Seoul agreed to deploy an advanced U.S. missile-defense system in South Korea—over strong opposition from China—to counter the threat from North Korea.

The agreement on a deployment by late next year was announced jointly Friday by Seoul’s defense ministry and a U.S. military command stationed in South Korea.

“North Korea’s continued development of ballistic missiles and weapons of mass destruction require the Alliance to take this prudent, protective measure to bolster our layered and effective missile defense,” Gen. Vincent K. Brooks, who commands combined U.S.-South Korean forces, said in a statement.

Last month North Korea test-fired a series of midrange ballistic missiles capable of hitting targets as distant as the U.S. territory of Guam and U.S. military facilities in Okinawa, Japan. That followed a nuclear test in January and a launch the next month of a missile with enough range to threaten the continental U.S.

The planned deployment of the Thaad system—for Terminal High-Altitude Area Defense—underscores the U.S. “ironclad commitment” to defend South Korea, the U.S. statement said. The two Koreas technically remain at war, the 1950-53 Korean War having ended with an armistice rather than a peace treaty. The U.S., which led the fight in that war after the North invaded, still stations 28,500 troops in the South.

After months of negotiations the U.S. and South Korean governments are in a final stage of selecting a location in the southern part of the peninsula, a spokesman at Seoul’s defense ministry said.

The issue of Thaad in South Korea has been sensitive beyond the peninsula—opposed not only by North Korea but also China and Russia, which see deploying the antimissile system in the region as undermining their own security. The U.S. and South Korea, apparently seeking to allay these concerns, reaffirmed in their official statements that the system “will be focused solely on North Korean missile threats” and “will not be directed towards any other third-party nations.”

Observers say Beijing is anxious about range of the system’s radar, capable of reaching deep into China.

Relations between North Korea and China, its biggest trading partner and aid donor—have soured over Pyongyang’s nuclear and missile tests—but analysts say Beijing still cherishes North Korea as a bulwark to keep U.S. power in the region from expanding to its doorstep. In recent months, Beijing and Pyongyang have worked to mend ties.

South Korea and China are linked by growing economic ties, but Seoul has said that given the immediate and constant threat from North Korea, it won’t compromise on public safety. The new U.S. missile-defense system is “a defensive measure to ensure the security of the South and its people” as well as to protect the alliance, said the defense ministry in Seoul.

Write to Kwanwoo Jun at kwanwoo.jun[a]wsj.com