NEW : U.S. says it `` stands vigilantly by '' U.S. . NEW : North Korea denies responsibility for hacking North Korean computers in Annual . U.S. will talk to Pyongyang , but Pyongyang has to talk giving up nukes , Kevin Bohn says . Ballistic missiles may be able to carry nukes , a U.S. defense intelligence assessment says .
NEW : U.S. says it `` stands vigilantly by '' South Korea . NEW : North Korea denies responsibility for hacking South Korean computers in March . U.S. will talk to North Korea , but Pyongyang has to talk giving up nukes , Kerry says . Ballistic missiles may be able to carry nukes , a U.S. defense intelligence assessment says .