The United States imposed sanctions Wednesday on 24 Chinese companies and several people who allegedly participated in building and militarizing disputed artificial islands in the South China Sea. The China did not immediately respond to the sanctions. China claims at least 80% of the South China sea, which is home to vast oil and gas reserves. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also claim jurisdiction to parts of the area through which trade valued at $3 trillion passes annually.   Among the sanctioned state-owned companies are construction giant China Communications Construction Co., a subsidiary of the China Shipbuilding Group, and a telecommunications company.