U.S. Pacific Command said today it has deployed to South Korea the “first elements” of a controversial Terminal High-Altitude Area Defense missile system for protection against increasing ballistic threats from North Korea.

The move, which has been planned for months, comes as the U.S. military detected and tracked multiple missile launches out of North Korea today, 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,” U.S. Pacific Command, based at Camp H.M. Smith on Oahu, said today in a release. “The deployment of THAAD contributes to a layered missile defense system and enhances the U.S.-(South Korea) alliance’s defense against North Korean missile threats.”

“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,” said Adm. Harry B. Harris Jr., commander of U.S. Pacific Command. “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, Pacific Command said.

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.

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,” China Foreign Ministry spokesperson Geng Shuang said today. “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 today in violation of U.N. Security Council resolutions.

The type of equipment sent to South Korea for the THAAD system wasn’t revealed. The entire system is expected to be deployed in phases.