Moon told U.S. Sen. Dick Durbin, D-Ill., that Seoul's fact-finding mission into four unreported THAAD missile launchers is a domestic issue that would not affect any bilateral decision, South Korean news service News 1 reported.
Durbin said he had heard about Moon's decision and had asked for the Korean president's opinion during the meeting, according to the report.
"I do not mean to change any existing decisions, or send a different message to the United States," Moon said, adding that while he does not take the bilateral decision lightly, an environmental impact assessment should be completed.
THAAD was deployed at a former golf course in Seongju, central South Korea, before the assessment was finished.
Durbin also met with Defense Minister Han Min-koo on Wednesday, where he expressed U.S. bipartisan support for the alliance with South Korea and for the denuclearization of North Korea, Yonhap reported.
Moon's call for a THAAD probe has triggered criticism from the opposition, including from the conservative Liberty Korea Party, the centrist People's Party and the center-right Bareun Party.
Moon's ruling Democratic Party of Korea supported his decision, calling the omitted report on four additional launchers a "serious breach of national order," News 1 reported.
Protesters continue to gather near the THAAD deployment site in Seongju, where they meet weekly to oppose the U.S. missile defense system, local news service Oh My News reported.
More than 100 protesters from Seongju and nearby Gimcheon gathered on Wednesday, calling for an investigation into the deployment, according to the report.