Japanese scientists have found "measurable" concentrations of radioactive iodine-131 and caesium-137 in seawater samples taken 30 km (18.6 miles) from land, the U.N. nuclear watchdog said on Thursday.

"The iodine concentrations were at or above Japanese regulatory limits, and the caesium levels were well below those limits," the International Atomic Energy Agency (IAEA) said.

Japanese authorities had given the Vienna-based agency data on samples collected on March 22-23, after detecting iodine and caesium in the water near the crippled Fukushima Daiichi nuclear power plant, it added in a statement.

"The IAEA's Marine Environmental Laboratory in Monaco has received the data for review," the IAEA said.