SEOUL, June 23 (Xinhua) -- South Korea's presidential Blue House said Friday that plan to deploy the U.S. Terminal High Altitude Area Defense (THAAD) had been revised to bring forward the date of delivery and installation in the country's southeastern region.

A senior Blue House official, who declined to be identified, told reporters that the THAAD deployment agreement had been revised in the meantime to install two THAAD mobile launchers earlier than originally agreed upon.

Seoul and Washington announced their joint agreement in July last year to install one THAAD battery in Seongju county, North Gyeongsang province.

Under the original agreement, one mobile launcher was scheduled to be installed by the end of this year, with five others set to be deployed in 2018.

One THAAD battery is composed of six mobile launchers, 48 interceptors, the AN/TPY-2 radar and the fire and control unit.

Actually two mobile launchers were transported to South Korea on March 6 and installed in the deployment site on April 26, just about two weeks before the country's presidential by-election on May 9.

Four more launchers were delivered to a U.S. military base near Seongju, but it was not reported to President Moon Jae-in, who later ordered a thorough investigation into the unreported transportation.

The process of mobile launcher installations was changed a couple of times, the Blue House official told reporters, saying President Moon also had no knowledge of why the changes were made as investigation was still underway.