The US Defense Department said Tuesday it has been "very transparent" in its deployment of the THAAD missile defense system to South Korea, after President Moon Jae-in expressed frustration and ordered an investigation into how additional launchers were brought in without his knowledge.

"We continue to work very closely with the Republic of Korea government and we have been very transparent in all of our actions throughout this process," Pentagon spokesman Navy Capt. Jeff Davis told reporters.

Earlier in the day, Moon said it was "very shocking" to learn that the Defense Ministry failed to report to him that an additional four launchers for THAAD have been brought into the country in addition to the two that had already been installed.

Moon also ordered a probe into the matter.

The ministry, however, claimed that it has reported the deployment of the additional launchers to Moon's national security adviser, Chung Eui-yong.

The THAAD deployment has been one of the most sensitive political and defense issues in South Korea for years as China has vehemently opposed the deployment, claiming that the system, especially its powerful X-band radar, hurts its security interests.

The deployment has become even more unpopular in South Korea as the public raised questions about decisions made by former President Park Geun-hye, who was impeached and then ultimately ousted from office on corruption charges. It was when Park was in office that Seoul decided to host the system.

Moon has also been critical of the decision, saying it was made without due process to collect public opinion.

US President Donald Trump fueled opposition to THAAD among the South Korean public by demanding Seoul pay for the $1 billion system. The demand runs counter to a deployment agreement under which the US agreed to pay for the system if the South agreed to host it and provide land. (Yonhap)