Japan's nuclear agency has upgraded the severity level of a radioactive water leak at the Fukushima plant from one to three on an international scale.

Highly radioactive water was found to be leaking from a storage tank into the ground at the plant on Monday.

It was first classified as a level one incident on the International Nuclear and Radiological Event Scale (Ines).

But Japan's Nuclear Regulation Authority proposes elevating it to level three on the seven-point scale.