TOKYO (Reuters) - Radiation at a tank holding highly contaminated water at Japan's crippled Fukushima nuclear plant has spiked 18-fold, the plant's operator said on Sunday.

Radiation of 1,800 millisieverts per hour - enough to kill an exposed person in four hours - was detected near the bottom of one storage tank on Saturday, Tokyo Electric Power Co, also known as Tepco, said.

An August 22 reading measured radiation of 100 millisieverts per hour at the same tank. A Tepco spokesman also said a new leak had been detected from a pipe connecting two other tanks.

The Fukushima plant was destroyed by a tsunami in March 2011 in the worst nuclear disaster since Chernobyl a quarter of a century earlier.

(Reporting by Tetsushi Kajimoto; Editing by Paul Tait)