Highly toxic radioactive strontium has been found in groundwater near the crippled Fukushima nuclear plant.

It is the first time the substance has been detected in groundwater near the plant's No. 1 and No. 2 reactors.

The operator of the Fukushima plant has also confirmed strontium up to 240 times the legal limit has been found in seawater near the facility.

Strontium tends to accumulate in bones and can cause bone cancer and leukaemia.

Japan's nuclear safety agency says it will now monitor the effects of the strontium on fish and marine life near the plant.

Last week, soil samples from outside the Fukushima plant also revealed concentrations of strontium.

Japan's NHK news reported strontium-90 had been found at 11 sites in Fukushima prefecture.

The substance is generated during the fission of uranium in fuel rods in nuclear reactors.

It is described as a bone seeker, accumulating in bone and bone marrow. It can cause cancer and leukaemia and has a half-life of 29 years.