The Terminal High Altitude Area Defense (THAAD) system battery was initially deployed in March in the southeastern region of Seongju with just two of its maximum load of six launchers to counter a growing North Korean missile threat.
During his successful campaign for the May 9 presidential election, Moon called for a parliamentary review of the system, whose deployment has also infuriated China, North Korea's lone major ally.
Related reading
North Korean media says ballistic missile test successful
"President Moon said it was very shocking" to hear the four additional launchers had been installed without being reported to the new government or to the public, presidential spokesman Yoon Young-chan told a media briefing.
Moon had campaigned on a more moderate approach to Pyongyang, calling for engagement even as the reclusive state pursues nuclear weapons and ballistic missile programs in defiance of U.N. Security Council resolutions and threats of more sanctions.
The US military in South Korea did not have immediate comment on Moon's comments. The South Korean military also did not immediately comment.
China tensions easing
Moon's order of a probe into the THAAD launchers came amid signs of easing tensions between major trading partners South Korea and China.
South Korea's Jeju Air said on Tuesday China has approved a plan to double its flights to the Chinese city of Weihai from June 2.
China has been incensed over the THAAD deployment, fearing it could give the US military the capability of seeing into its own missile systems, and could open the door to a wider deployment of the system, possibly in Japan and elsewhere, military analysts say.
Related reading
Moon warns 'high possibility' of clashes with North Korea
China has denied it had discriminated against South Korean companies, which have faced product boycotts and bans on Chinese tourists visiting South Korea.
A Korean-Chinese joint drama “My Goddess, My Mom" starring South Korean actress Lee Da-hae, whose broadcast had been indefinitely delayed in China, was told by its Chinese partner recently that it will soon be aired, according to JS Pictures, Lee's agent.
An official at South Korean tour agency Mode Tour told Reuters it hoped China may lift a ban on selling trips to South Korea, which had been in place since March 15, as early as the second week of June.
Although there have been no official orders from the Chinese government to lift the ban, a few Chinese travel agencies have sent inquiries about package tours, he said. However, Lotte Group has yet to reopen any of the 74 retail stores in China it was forced to close in March after the group allowed South Korea to install the THAAD system on land it owned.
Bomber drill
The United States, which has 28,500 troops stationed in South Korea, has a mutual defense treaty with Seoul dating back to the end of the 1950-53 Korean War, which ended in a truce that has left the peninsula in a technical state of war.
South Korea's Defence Ministry said on Tuesday it had conducted a joint drill with a US supersonic B-1B Lancer bomber on Monday. North Korea's state media had earlier accused the United States of staging "a nuclear bomb dropping drill".
Japanese Prime Minister Shinzo Abe talked to Moon by phone on Tuesday, Japan's Foreign Ministry said in a statement, and both agreed that North Korea's continued provocations were unacceptable.
Abe told Moon that dialogue for dialogue's sake with North Korea would be meaningless, and that China's role in exerting pressure on the North was important.
Related reading
New South Korean president looks to new direction
The North's KCNA news agency reported leader Kim Jong Un supervised Monday's test of a missile equipped with a new precision guidance system and an improved pre-launch automated sequence and a new mobile launch vehicle.
Kim said North Korea would develop more powerful weapons in multiple phases in accordance with its timetable to defend North Korea against the United States.
"He expressed the conviction that it would make a greater leap forward in this spirit to send a bigger 'gift package' to the Yankees" in retaliation for American military provocation, KCNA quoted Kim as saying.
Related reading
THAAD system operational in South Korea: US
