The substances were detected in seawater which was sampled yesterday about 100 metres south of the Fukushima No 1 plant, a Tokyo Electric Power Co (TEPCO) official said, stressing it was not a threat to human health.
"Normally, such radioactive substances are not detected in the area," said Naoki Tsunoda, adding that the company will continue monitoring at the same point and in other areas.
TEPCO said the level of iodine-131 was 126.7 times higher and caesium-134 was 24.8 times higher than government-set standards.
The level of caesium-137 was also 16.5 times higher while that of cobalt-58 was lower than the standard, said Tsunoda.
A 9.0-magnitude quake and ensuing tsunami on March 11 devastated Japan''s northeastern Pacific coast, knocking out the plant's cooling systems and leaving it on the brink of a catastrophic meltdown.
Helicopters and fire trucks have been deployed to pour water over heating fuel rods at the plant since Thursday.