SEOUL, South Korea — On his first mission to reassure an important American ally, Defense Secretary Jim Mattis met on Thursday with top South Korean officials, who agreed to push ahead with the deployment of a new missile defense system.

“Thaad is for defense of our allies’ people, of our troops who are committed to their defense,” Mr. Mattis told reporters, using the acronym for Terminal High-Altitude Area Defense, the American antimissile system. It is meant to intercept North Korea’s medium-range missiles.

“Were it not for the provocative behavior of North Korea, we would have no need for Thaad out here,” Mr. Mattis added. “There is no other nation that needs to be concerned about Thaad.”

South Korea was a logical first stop for Mr. Mattis, who will also visit Japan on the trip. Tensions have risen in the region after Kim Jong-un, the North Korean leader, proclaimed during his New Year’s Day address that his military was preparing to conduct its first test launch of an intercontinental ballistic missile.