North Korea accused South Korea's navy Tuesday of trespassing in its waters and threatened military action, further raising tensions sparked by the sinking of one of Seoul's warships in March. The South has announced a series of reprisals including a trade ban after a multinational investigation concluded last week that a North Korean submarine had torpedoed the Cheonan on March 26. The communist North denies involvement in the sinking, which claimed 46 lives and brought it widespread international condemnation. It threatens full-scale war if there is any attempt to punish it.