Hagel made a symbolic stop at the border town of Panmunjom, where the divided Koreas signed a truce in 1953, Yonhap news service reported.
After shaking hands with soldiers at a U.S. military base, the secretary walked into the 2.5-mile-wide no-man's land to tour a small post less than 100 feet from the official border of North Korea.
"This is a very important location for our relations, and probably there is a clear identification of the ROK [Republic of Korea]-U.S. partnership here more than anywhere else," Hagel told reporters.
Hagel is in South Korea ahead of a defense summit on Wednesday that is scheduled to assess continuing threats from Pyongyang.
It is his first trip to the country since taking office in February.
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