HAVANA — Cuba's explanation that it buried antiquated weapons systems under thousands of tons of sugar and sent them back to North Korea for repair is potentially credible but leaves troubling questions unresolved, international arms experts said Wednesday. Acting on intelligence it hasn't publicly described, Panama seized the rusting, 34-year-old North Korean freighter Chong Chon Gang on July 11 as it headed toward the Caribbean entrance of the Panama Canal on its way to the Pacific and its final destination of North Korea. Hidden under some 240,000 white sacks of raw brown Cuban sugar, Panamanian officials found shipping containers with parts of a radar system for a surface-to-air missile defense system, an apparent violation of U.N. sanctions that bar North Korea from importing sophisticated weapons or missiles.