SEOUL, South Korea (AP) — North Korean leader Kim Jong Un said his past negotiations with the United States only confirmed Washington’s “unchangeable” hostility toward Pyongyang and described his nuclear buildup as the only way to counter external threats, state media said Friday. Kim spoke Thursday at a defense exhibition where North Korea displayed some of its most powerful weapons systems, including intercontinental ballistic missiles designed to target the U.S. mainland, the North’s Korean Central News Agency said. While meeting with army officers last week, he had pledged a “limitless” expansion of his military nuclear program.