North Korea stopped operating a radio station used to send coded messages to its agents in South Korea, the Yonhap news agency said on Saturday, the latest sign the isolated country is shaking up the way it handles relations with Seoul. North Korea has been stepping up pressure on Seoul in recent weeks, declaring it the "principal enemy", saying the North will never reunite with the South and vowing to enhance its ability to deliver a nuclear strike on the U.S. and America's allies in the Pacific. Radio Pyongyang, known as a numbers station, in the past broadcast mysterious coded numbers presumed to be targeted at Pyongyang's spies operating in South Korea.