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A South Korean soldier from 6th Engineer Brigade participates in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649185 )

A U.S. Army M1A2 tank crosses Nam Han river on a South Korean military barge during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. According to U.S. military authorities, the training is aimed at enhancing the two allies' combined combat capabilities. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

U.S. Army's M1A2 tank and soldiers from 2nd Battalion, 9th Infantry Regiment of the 1st Armored Brigade Combat Team of 2nd infantry division and South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649210 )

South Korean soldiers transport a U.S. 2nd Infantry Division tank on a pontoon bridge of the South Korean Army during their joint river crossing operation drill in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. The two Koreas remain technically at war under a mere truce that ended hostilities in their 1950-53 conflict. REUTERS/Lee Jae-Won ( KOREA/ )

A U.S. Army M1A2 tank crosses Nam Han river on a South Korean military barge during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

U.S. Army's M1A2 tank and soldiers from 2nd Battalion, 9th Infantry Regiment of the 1st Armored Brigade Combat Team of 2nd infantry division participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649176 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649179 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649188 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649189 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649193 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649196 )

U.S. soldiers from 2nd Battalion, 9th Infantry Regiment of the 1st Armored Brigade Combat Team of 2nd infantry division participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649199 )

A South Korean soldier from 6th Engineer Brigade participates in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649200 )

U.S. Army's M1A2 tank and soldiers from 2nd Battalion, 9th Infantry Regiment of the 1st Armored Brigade Combat Team of 2nd infantry division and South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649202 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649204 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649205 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649206 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649207 )

South Korean soldiers from 6th Engineer Brigade participate in a river crossing exercise on May 30, 2013 in Yeoncheon-gun, South Korea. (Photo by Chung Sung-Jun/Getty Images) ( 169649209 )

South Korean soldiers transport a U.S. 2nd Infantry Division tank on a pontoon bridge of the South Korean Army during their joint river crossing operation drill in Paju, near the demilitarised zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

South Korean soldiers transport a U.S. 2nd Infantry Division tank on a pontoon bridge of the South Korean Army during their joint river crossing operation drills in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

South Korean soldiers transport a U.S. 2nd Infantry Division tank on a pontoon bridge of the South Korean Army during their joint river crossing operation drills in Paju, near the demilitarised zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

South Korean soldiers assemble a pontoon bridge during a joint river crossing operation drill with U.S. troops in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

South Korean soldiers assemble a pontoon bridge during a joint river crossing operation drill with U.S. troops in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

South Korean soldiers stand guard as a U.S. 2nd Infantry Division tank gets ready during their joint river crossing operation drill in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. REUTERS/Lee Jae-Won ( KOREA/ )

A South Korean soldier adjusts flags of the U.S. and South Korea during a joint river crossing operation drill in Paju, near the demilitarized zone separating North Korea from the South, north of Seoul May 30, 2013. T REUTERS/Lee Jae-Won ( KOREA/ )

A U.S. Army M1A2 tank sits on a South Korean military barge after crossing Nam Han river during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

A soldier directs a U.S. Army M1A2 tank after it crossed Nam Han river during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

South Korean army soldiers train on a floating bridge during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

U.S. and South Korean Army soldiers cross Nam Han river during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

South Korean army soldiers work to set up a floating bridge during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )

A South Korean army soldier guides an armored vehicle through smoke during a joint military exercise between the U.S. and South Korea in Yeoncheon near the border with North Korea, in South Korea, Thursday, May 30, 2013. (AP Photo/Ahn Young-joon) ( South Korea US Military Exercise )