The level of radioactive iodine in the sea off Japan's disaster-hit Fukushima nuclear plant has soared to its highest reading yet at 4.385 times the legal limit, the plant operator said.

According to AFP, the level of iodine-131, reported a few hundred meters south of its southern water outlet has risen in a series of tests since last week, carried out by plant operator the Tokyo Electric Power Company (TEPCO).

Previous readings there were 1.250 times the legal maximum on Friday, March 25, 1.850 times the limit on Saturday, March 26, and 3.355 times the limit on Tuesday, March 29.

A 9.0 magnitude earthquake and tsunami on March 11 knocked out the cooling systems of the Fukushima plant's six reactors - triggering explosions and fires, releasing radiation and sparking global fears of a widening disaster.