WASHINGTON - The United States tightened its economic crackdown on North Korea over its nuclear arms program on Wednesday, imposing sanctions on nine entities, 16 people and six ships it accused of helping Pyongyang develop weapons of mass destruction.The US Treasury said it imposed sanctions on two China-based trading firms involved in exporting millions of dollars worth of metals and other goods used in North Korea's weapons industry.The US Treasury also imposed sanctions on officials belonging to the Workers Party of Korea who are operating in China, Russia and Georgia's Abkhazia region and urged those countries to expel the individuals.