SEOUL (Reuters) - A North Korean soldier defected to South Korea on Thursday, the South’s military said, making a rare crossing of one of the world’s most heavily fortified borders amid heightened tension between the rival neighbors. The soldier crossed the military demarcation line that runs through the demilitarized zone dividing the two Koreas, which remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty. While on average more than 1,000 North Koreans defect to South Korea every year, most travel via China and it is unusual for a North Korean to cross the DMZ, which is heavily mined and lined with barbed wire and soldiers on both sides. The last such crossing was in June 2015. The soldier was unarmed and there was no exchange of fire, a South Korean military official said. He walked across an eastern part of the front line at around 10 a.m.