Seoul has accused North Korea of hacking into the smartphones of dozens of South Korean officials and stealing text and voice messages.Tuesday's announcement came a day after North Korea warned of pre-emptive nuclear strikes in response to the start of Seoul-Washington military drills it views as an invasion rehearsal.Seoul's National Intelligence Service said in a statement that the cyberattacks were made between late February and early March. It didn't say which officials' phones were hacked or if there was any sensitive information in the stolen text and voice messages.There was no immediate reaction from North Korea.Seoul has accused Pyongyang of launching a slew of cyberattacks in recent years. Pyongyang has denied the claims.