Two launchers are already in operation at the THAAD site in Seongju, North Gyeongsang Province, some 300 kilometers southeast of Seoul.

Moon said it's "very shocking" that the ministry has not formally reported to him on the extra THAAD elements brought into South Korea, according to presidential press secretary Yoon Young-chan.

Moon, who took office on May 10, apparently confirmed the THAAD equipment this week.

He ordered a probe into what Cheong Wa Dae views as negligence of duty by local defense officials.

"An investigation is under way on the president's comments. It's a matter to see the results," Han told reporters. "There's no more to say."

Repeatedly asked to give some comment, the minister said, "Let me speak later on."

Ministry officials earlier said they briefed a presidential security aide last week on the four THAAD launchers.

South Korea and the US agreed last year to deploy a THAAD unit here to counter North Korea's nuclear and missile threats.

The deal was reached under South Korea's previous administration of Park Geun-hye and Han was appointed by Park.

Moon, a liberal leader, has been critical of the decision, saying it lacked transparency and did not take into consideration public opinion.

A THAAD battery consists of six truck-mounted launchers, fire control and communication equipment, and an advanced X-band radar. (Yonhap)