U.S. President Barack Obama recently sent a thank-you letter to the South Korean commander in the Joint Security Area who provided security during his visit to the demilitarized zone in March.

"This is the first time a U.S. president has sent a letter to encourage a Korean military officer in the field," a military official said.

The letter, dated July 19 and signed by Obama, was sent to Army Lieutenant General Yoon Bong-hee. Obama thanked him for all the hospitality and support during his trip there.

When he came to Korea to attend the Nuclear Security Summit in Seoul last March, Obama visited South Korea's northernmost post in the DMZ and observed North Korea through a pair of binoculars.

"I never expected to get a letter from him; that's so thoughtful. I'd like to express my thanks to him," Yoon said.