The United States on Wednesday blacklisted two North Korean shipping firms, Chongchongang Shipping and Ocean Maritime Management, following a similar move by the United Nations. The United Nations on Monday said Ocean Maritime Management operated a ship detained a year ago near the Panama Canal for smuggling Soviet-era arms, raising concerns about Cuba’s military cooperation with North Korea. The new sanctions, published on the Treasury Department’s website, freeze any assets the companies may hold in the United States and prohibit people and firms in the United States from dealing with them.