Soldiers of the U.S. Army 23rd chemical battalion wear gas masks while attending a demonstration of their equipment during a ceremony to recognize the battalion's official return to the 2nd Infantry Division based in South Korea at Camp Stanley in Uijeongbu, north of Seoul, Thursday, April 4, 2013. The 23rd chemical battalion left South Korea in 2004 and returned with some 350 soldiers in Jan. 2013. (AP Photo/Lee Jin-man)
Soldiers of the U.S. Army 23rd chemical battalion wear gas masks while attending a demonstration of their equipment during a ceremony to recognize the battalion's official return to the 2nd Infantry Division based in South Korea at Camp Stanley in Uijeongbu, north of Seoul, Thursday, April 4, 2013. The 23rd chemical battalion left South Korea in 2004 and returned with some 350 soldiers in Jan. 2013. (AP Photo/Lee Jin-man)
** FILE ** In this June 27, 2008, file photo released by China's Xinhua News Agency, the cooling tower of the Yongbyon nuclear complex is demolished in Nyongbyon, also known as Yongbyon, North Korea, in a sign of its commitment to stop making plutonium for atomic bombs. North Korea vowed Tuesday, April 2, 2013, to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats against the United States and South Korea. (AP Photo/Xinhua, Gao Haorong, File)
U.S. Secretary of State John Kerry speaks at a news conference with South Korean Foreign Minister Yun Byung-Se, not pictured, at the State Department in Washington, on Tuesday, April 2, 2013. (AP Photo/Jacquelyn Martin)
** FILE ** Park Geun-hye (center), South Korea's new president, arrives for the official dinner at the presidential Blue House in Seoul on Monday, Feb. 25, 2013. Ms. Park took office as the country's first female president earlier in the day. (AP Photo/Kim Jae-hwan, Pool)
** FILE ** A South Korean army officer stands guard at the Inter-Korean Transit Office in Paju, South Korea, near North Korea's border city of Kaesong, on Sunday, March 15, 2009. (AP Photo/Ahn Young-joon)
South Korean soldiers patrol along a barbed-wire fence in Paju, South Korea, near the border village of Panmunjom, on Sunday, March 31, 2013. North Korea warned South Korea on Saturday that the Korean Peninsula had entered "a state of war" and threatened to shut down a border factory complex that's the last major symbol of inter-Korean cooperation. (AP Photo/Ahn Young-joon)
** FILE ** In this June 27, 2008, file photo released by China's Xinhua News Agency, the cooling tower of the Yongbyon nuclear complex is demolished in Nyongbyon, also known as Yongbyon, North Korea, in a sign of its commitment to stop making plutonium for atomic bombs. North Korea vowed Tuesday, April 2, 2013, to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats against the United States and South Korea. (AP Photo/Xinhua, Gao Haorong, File)
** FILE ** In this June 27, 2008, file photo released by China's Xinhua News Agency, the cooling tower of the Yongbyon nuclear complex is demolished in Nyongbyon, also known as Yongbyon, North Korea, in a sign of its commitment to stop making plutonium for atomic bombs. North Korea vowed Tuesday, April 2, 2013, to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats against the United States and South Korea. (AP Photo/Xinhua, Gao Haorong, File)
