The U.S. ambassador to South Korea said Thursday there is nothing his country could have done differently to make North Korea give up its nuclear program. Sung Kim was named ambassador last month after serving as the chief U.S. envoy in the six-party talks. He met with reporters in Seoul as diplomatic sources said the United States and North Korea planned more bilateral discussions. \"I\'ve really thought very hard what we could have done differently, but I\'m afraid, I just can\'t come up with a magic answer,\" Kim said. \"But it\'s really the North Koreans, they need to demonstrate their serious commitment, they need to take the actions necessary to reopen the six-party talks.\" The six-party talks involve both Koreas, the United States, Russia, China and Japan. The first series of talks was held in 2003, and the sixth and so far the last round ended in 2007 without any agreement.