The Hawaii-based U.S. Pacific Command said Monday it had deployed to South Korea the “first elements” of a controversial Terminal High-Altitude Area Defense system for protection against growing ballistic missile threats from North Korea.

The move, which has been planned for months, comes as the U.S. military detected and tracked multiple missile test launches out of North Korea on Monday, with four of the medium-­range rockets landing in the Sea of Japan, according to the Pentagon.

“North Korea’s accelerating program of nuclear weapons tests and ballistic missile launches constitute a threat to international peace and security, and are in violation of multiple United Nations Security Council resolutions,” the command, based at Camp H.M. Smith on Oahu, said in a news release. “The deployment of THAAD contributes to a layered missile defense system and enhances the U.S.-(South Korean) alliance’s defense against North Korean missile threats.”

ADM. Harry B. Harris Jr., commander of U.S. Pacific forces, added, “Continued provocative actions by North Korea, to include (this) launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy THAAD to South Korea. We will resolutely honor our alliance commitments to South Korea and stand ready to defend ourselves, the American homeland and our allies.”

The THAAD system is a defensive capability and is designed to intercept and destroy short- and medium-range ballistic missiles inside or outside the atmosphere during their final, or terminal, phase of flight, the command said.

“Work continues to operationalize THAAD as quickly as possible,” U.S. Forces Korea said in a release.

The North Korean missile launches followed the start last week of Foal Eagle 2017, with about 3,600 U.S. service members joining the 28,000 U.S. troops already in South Korea for the annual exercise. The aircraft carrier USS Carl Vinson and new F-35 Joint Strike Fighters will take part in the exercise with about 300,000 South Korea soldiers, the Chosun Ilbo newspaper reported.

In typical bluster, North Korea last week said it would “deal a merciless retaliatory strike” at any provocation, and pledged to strengthen its nuclear deterrent, Yonhap News said.

South Korea already has Patriot PAC-2 batteries and was upgrading to PAC-3 for defense against lower-altitude missiles, with THAAD expected to provide an upper tier of defense.

“Adding THAAD to missile-defense deployments that already include Patriot systems would likely substantially enhance South Korea’s capacity to minimize the damage caused by a large North Korean missile attack,” according to the website 38 North, a program at the Korea Institute at the Johns Hopkins School of Advanced International Studies.

Guam has its own THAAD system to protect against North Korean intermediate-range missiles. China is opposed to the THAAD system in South Korea because of its potential to track missile launches from China.

”We believe that the deployment disrupts strategic balance of the region, undermines strategic and security interests of countries in the region including China, and does harm to peace and stability of the Korean Peninsula,” Chinese Foreign Ministry spokesman Geng Shuang said Monday. “We urge relevant parties to face squarely China’s reasonable concerns, call off the deployment and avoid traveling further down the wrong path.”

China also said it was opposed to the North Korean missile launches in violation of U.N. Security Council resolutions. The type of equipment sent to South Korea for the THAAD system wasn’t revealed, with the system expected to be deployed in phases.