Adm. Harry B. Harris Jr., the head of the United States Pacific Command, announced the start of the deployment, saying that “continued provocative actions by North Korea, to include yesterday’s launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy Thaad to South Korea.”

The developments come as South Korea is consumed by turmoil over the impeachment of President Park Geun-hye, whose administration agreed to the Thaad deployment. But with the president facing possible removal from office over a corruption scandal, the fate of the system had been in doubt. Its accelerated deployment could make it harder, if not impossible, for her successor to head off its installation.

Moon Jae-in, an opposition leader who is the front-runner in the race to replace President Park, acknowledged that it would be difficult to overturn South Korea’s agreement to deploy the system. But he has insisted that the next South Korean government should have the final say on the matter, saying that Ms. Park’s government never allowed a full debate on it.

Last year, thousands of people in Seongju, a rural southern county in South Korea, protested when it was announced that a Thaad battery would be established there. They said they feared that the system would harm their agricultural livelihoods. Many South Koreans also worry that any expansion of military ties with the United States could worsen already festering tensions with North Korea and China.

Under its deal with Washington, South Korea is providing the land for the missile system and will build the base, but the United States will pay for the system, to be built by Lockheed Martin, as well as its operational costs.

The United States military statement said that “the first elements” of Thaad were deployed on Monday, the same day as the North’s missile launchings.

A C-17 cargo plane landed at the United States military’s Osan Air Base, about 40 miles south of Seoul, on Monday evening, carrying two trucks, each mounted with a Thaad launchpad. More equipment and personnel will start arriving in the coming weeks, South Korean military officials said.