SEOUL, South Korea — President Moon Jae-in of South Korea ordered an investigation on Tuesday into why the Defense Ministry had failed to inform him that four additional launchers for a contentious American missile-defense battery had been brought into his country.

The United States hurried to deploy the antimissile system, called the Terminal High Altitude Area Defense, or Thaad, in late April despite protests from Mr. Moon, a presidential candidate at the time, who demanded that the deployment be delayed until after the new government in Seoul reviewed whether it was in the country’s best interest.

The president’s move came as tensions on the Korean Peninsula were rising. On Monday, Pyongyang launched what South Korean officials called a Scud-type missile that flew 280 miles before falling into waters near Japan. Hours after the missile test, two United States supersonic B-1B Lancer bombers conducted a drill over South Korea, the second such mission this month.

As a candidate, Mr. Moon had called for a parliamentary review of the Thaad system. Two interceptor-rocket launchers were first put in place in Seongju, 135 miles southeast of Seoul, during the April deployment, although a Thaad battery normally operates with six launchers. Mr. Moon had accused both Washington and the government of the impeached President Park Geun-hye of trying to make the Thaad deployment a fait accompli.