Typhoon Songda weakened into a depression as it churned along Japan's Pacific coast Sunday but was still expected to bring heavy rains to the northeast which was hit by a massive earthquake and tsunami.

A total of 57 people suffered storm-related injuries on the southern Okinawan islands, police said. Of those, five were seriously hurt.

Songda was downgraded to a "temperate depression" by the Japan Meteorological Agency as it was located off the southern main island of Shikoku at around 3:00 pm (0600 GMT).

Moving northeast at a speed of 55 kilometres (35 miles) per hour, it is expected to reach Tokyo and its vicinity at around 9:00 am (0000 GMT) Monday, said agency official Takeo Tanaka.

"But it is feared to stimulate the rain front over the Pacific coast of Honshu island, including the disaster-hit areas," he said.

Up to about 15 centimetres (six inches) of rainfall was forecast for the northeast region over two days, the agency said.

It was not clear whether it would directly hit the Fukushima Daiichi nuclear power plant, more than 200 kilometres northeast of the capital, which was crippled by the March 11 natural disasters.

But the typhoon has already brought heavy rain to the Fukushima region, prompting fears that run-off water may wash away radioactive materials from land into the Pacific Ocean.

Plant operator Tokyo Electric Power Co. (TEPCO) has been pouring synthetic resins over the complex to prevent radioactive deposits from being swept away by winds or rain.