SINGAPORE--American workers hold key positions throughout China's domestic chip industry, helping manufacturers develop new chips to catch up with foreign rivals. Now, those workers are in limbo under new U.S. export control rules that prohibit U.S. citizens from supporting China's advanced chip development. At least 43 senior executives working with 16 publicly listed Chinese semiconductor companies are American citizens, according to an examination of company filings and official websites by The Wall Street Journal. Many of them hold C-suite titles, from chief executive to vice president and chairman. Almost all of the executives moved to China's chip industry after spending years working in Silicon Valley for U.S. chip makers or semiconductor equipment firms, according to the companies' filings.