The operator of Japan's crippled nuclear plant has suspended some of its outdoor work due to a tropical storm, just days after it admitted it was not prepared for harsh weather.
Heavy rain and strong winds are hitting north-east Japan, which was devastated in the 11 March earthquake and tsunami.
There are fears that more radioactive material from the Fukushima plant could drain into the land and sea.
Japan's Meteorological Agency has warned of mudslides and floods.
Typhoon Songda weakened to a tropical storm over south-west Japan late on Sunday, but strong winds and rain have continued to pound the north-east of the country.
Tokyo Electric Power Company (Tepco), which runs the nuclear plant, said it was on alert to ensure that contaminated water in reactor buildings did not flow out.
The level of water in the basement of one of the six reactor buildings rose by nearly 20cm (8in) in 24 hours to nearly 6m (20ft), Tepco said early on Monday.
"We presume the level of water has risen due to the rainwater which has seeped into the ground," said spokesman Junichi Matsumoto.
Workers have been spraying thousands of tonnes of water onto the damaged reactors to prevent fuel rods from overheating, after vital cooling systems were knocked out by the tsunami.
The emergency measures have left four reactor buildings with radioactive contaminated water pooling inside.
As a precaution, Tepco said it had also stopped spraying chemicals that it was using to prevent radioactive dust from spreading.
Tepco has been pouring anti-scattering agents - such as synthetic resins - around the damaged buildings of reactors one and four.
Separately, Tepco says two workers may have been exposed to unsafe levels of radiation.
The two men, who are in their 30s and 40s, have been working on and off at the plant since the twin natural disaster and may have been cumulatively exposed to radiation exceeding the official safe limit.
A company official said that the men were ''not at a stage that would require emergency medical treatment".
A special adviser to Prime Minister Naoto Kan criticised Tepco on Saturday, saying that the current safety measures at the plant "cannot be said to be appropriate".
Tepco and Japan's government have faced widespread criticism - both at home and abroad - over their handling of the Fukushima crisis.