SEOUL, South Korea — The United States and South Korea began installing key elements of an advanced missile-defense system in a rural southern county on Wednesday, a day after North Korea held huge artillery drills to mark the 85th anniversary of the founding of its military.

Work on the Terminal High Altitude Area Defense system, also known as Thaad, got underway in Seongju, 135 miles southeast of Seoul, while China, which strongly opposes the Thaad system, continued to try to tamp down tensions over signs that North Korea was preparing for a nuclear test.

It remained unclear whether the installation was ahead of schedule; Washington and Seoul have said only that they want Thaad operational “soon” or “within this year.”

The deployment has become a key issue in the campaign for the May 9 election to choose the successor to former President Park Geun-hye, who had agreed to the deployment but was forced from office in disgrace last month.