(Reuters) - U.S. stock index futures fell on Monday as growing trade tensions between the United States and China kept investors on edge, with the latest threat involving a check on Chinese investments in U.S. technology firms. The S&P 500 mini futures dropped 0.5 percent after a government official, with knowledge of the matter, said the U.S. Treasury Department was drafting curbs that would block firms with at least 25 percent Chinese ownership from buying U.S.