TOKYO, Sept 30 (Reuters) - Japan's Nikkei stock average logged its biggest drop in six weeks on Monday as investors went into risk-off mode and took profits on recent gainers amid growing concerns over a possible U.S. government shutdown. The benchmark Nikkei shed 2.1 percent to 14,455.80, its biggest one-day fall since Aug. 20. But the index rose 8 percent this month, posting its first monthly gain in five, and is up 39 percent this year. The broader Topix dropped 1.9 percent to 1,194.10. Trading volume was at a three-week low, with 2.70 billion shares changing hands. Market participants were also awaiting an announcement by Prime Minister Shinzo Abe on the government's economic growth and tax strategy on Tuesday.