South Korean President Moon Jae-in on Tuesday ordered a probe into the introduction of four Terminal High Altitude Area Defence (THAAD), missile launchers in addition to the two deployed by the U.S. military before his election. Presidential spokesman Yoon Young-chan told a media briefing that Moon was “shocked” to hear that the four additional THAAD launchers, deployed to counter the North Korean missile threat, were brought in without being reported to the new government or to the public,

The deployment of the defence system by the U.S. military, agreed by the government of Moon’s predecessor, was a controversial issue in the May 10 presidential election and has infuriated China, North Korea’s lone major ally. NAN reports that on May 2, China called for an immediate stop to the deployment of the THAAD anti-missile system to South Korea and is ready to protect its interests. Foreign Ministry spokesperson Geng Shuang voiced the government’s position against the move during a briefing. “We oppose the deployment of the U.S. missile system to South Korea and call on all parties to immediately stop this process. We are ready to take necessary measures to protect our interests,” he said, adding that “China’s position on the THAAD issue has not changed.”

The spokesperson didn’t specify what protective measures China had in mind. However, responding to the THAAD installation, China announced that it will stage live-fire exercises and test new weapons to protect its security. Beijing has previously voiced concerns over the THAAD system and joint U.S.-South Korean drills near the Korean Peninsula, consistently urging all the parties involved to find a peaceful solution to the volatile situation in the region.

Backed by Russia, it also proposed a halt to military drills in exchange for an end to Pyongyang’s missile and nuclear tests during a UN Security Council session held in New York. Moscow considers the stationing of the THAAD system to be an “additional destabilising factor for the region” amid alarmingly increasing tensions. It has called on Washington and Seoul to reconsider the decision.

Recently installed in South Korea, the THAAD system is aimed at detecting and shooting down missiles.