TOKYO, March 16 (Reuters) - Japan's Nikkei share average slipped on Friday in choppy trade as concerns about U.S. political uncertainty sapped investors' risk appetite, although the index managed to post weekly gains.
The Nikkei ended 0.6 percent lower at 21,676.51 after it briefly flirted in positive territory in early trade. For the week, the benchmark index gained 1.0 percent.
The dollar was 0.4 percent lower at 105.940 yen following a report by the Washington Post that U.S. President Donald Trump has decided to remove H.R. McMaster as his national security advisor.
Also souring sentiment was news that U.S. Special Counsel Robert Mueller has subpoenaed the Trump Organization for documents, including some related to Russia, the New York Times reported on Thursday, citing two people briefed on the matter.
Analysts said that uncertainty over U.S. political matters is casting a shadow over investment in risky assets, however, the Bank of Japan's exchange-traded fund purchases are expected to limit the downside.
The central bank bought 73.5 billion yen of ETFs each day on Wednesday and Thursday.
Index-heavy stocks underperformed, with Fanuc Corp shedding 1.6 percent, TDK Corp stumbling 3.3 percent and Tokyo Electron declining 1.4 percent.
Utility shares were also sold. Kansai Electric Power Co dropped 2.4 percent and Chubu Electric Power Co fell 1.3 percent.
The broader Topix dropped 0.4 percent to 1,736.63.
(Reporting by Ayai Tomisawa; Editing by Sam Holmes)