TOKYO - Tokyo stocks fell Monday as North Korea's detonation of what it claimed was a miniaturised hydrogen bomb spooked investors, sending regional tensions soaring.

The test comes less than a week after North Korea fired a rocket over Japan, with Washington on Sunday warning it could launch a "massive military response" to the latest provocation. Jittery traders bought the yen -- a safe haven currency in times of turmoil -- which dents the outlook for Japanese exporters' profitability and weighs on their shares.

The dollar bought 109.41 yen against 110.28 yen in New York on Friday. "The theme for the stock market continues to be geopolitical risks," Hiroaki Hiwada, a strategist at Toyo Securities, told Bloomberg News. Tokyo's benchmark Nikkei 225 index lost 0.93 percent, or 183.22 points, to close at 19,508.25, ending three days of gains, while the Topix index of all first-section issues fell 0.99 percent, or 16.04 points, to 1,603.55.

Electronics firms fell, with Panasonic off 1.70 percent at 1,437.5 yen and Hitachi falling 1.59 percent to 742 yen. Banks were also lower.

Mitsubishi UFJ fell 1.16 percent to 667.7 yen and rival Sumitomo Mitsui was down 1.09 percent at 4,075 yen.