By Adam Jourdan SHANGHAI (Reuters) – A tense stand-off between the United States and China over state-backed cyber espionage has dragged China’s secretive hacking unit “61398” back into focus, after the military group was pinpointed last year for mounting cyber attacks on Western commercial targets. U.S. authorities on Monday charged five Chinese military officers at the unit, accusing them of hacking into American nuclear, metal and solar firms to steal trade secrets. China on Tuesday summoned the U.S. ambassador in Beijing and warned it would retaliate if Washington followed through with the charges. At the center of the row is a nondescript tower block in the northern suburbs of China’s financial capital Shanghai, home to Chinese People’s Liberation Army (PLA) Unit 61398.