South Korea has said it would send more troops and guns to frontline islands, as North Korea warned it could follow up this week's deadly shelling with more attacks.Pyongyang's fresh warning came as a US aircraft carrier headed for the tense peninsula to join war games to be staged as a show of force to the nuclear-armed communist state. The government also said it would revise its rules of engagement, allowing troops to hit back harder if necessary. Meanwhile the Defence Minister, Kim Tae-young resigned, taking responsibility for a recent series of incidents. Reports said his resignation has been accepted by President Lee Myung.