The level of radioactive iodine-131 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, plant operator TEPCO said Thursday.

Last Friday the level of radioactive iodine in the seawater south of Fukushima was 1,250 times the limit which rose to 3,355 times on Tuesday, the AFP reported.

In the meantime, very low levels of radioactive iodine was found in a sample of milk from the West Coast state of Washington, the Environmental Protection Agency and the Food and Drug Administration said Wednesday, while the Chinese officials said “extremely low” levels of radiation from Japan's crippled nuclear power plant have spread to most Chinese provinces but remain far too low to be a health risk.

Another strong magnitude 6 earthquake hit northeast part of Japan, Japan Meteorological Agency reported Friday.

Epicenter of the quake was offshore near the Miyagi prefecture, at a depth of 40 kilometers, the agency said.

No tsunami warnings were issued.