Levels of radioactive iodine in the sea near the Fukushima nuclear plant are eight times higher than a week ago.
While officials say the radiation will no longer be a risk after eight days, it is a cause for a concern because it is unclear where the leak is coming from.
There are areas of radioactive water in four of the reactors at the plant, and two workers are in hospital.
The plant's operator says the core of a reactor may have been damaged, the BBC reports.
It has announced that fresh water rather than sea water will now be used to cool the damaged reactors, in the hope that this will be more effective.
The country's Prime Minister, Naoto Kan, says the situation at the quake-damaged Fukushima nuclear plant remains very precarious.
He said on Friday that the situation is not getting any worse, but it is not a time for complacency.
In a televised address, Mr Kan said the situation was very unpredictable and extreme vigilance was needed.
The operator of the plant has also warned that work to stabilise it may take another month.
Tests have showed that water in reactor 3 had radiation levels 10,000 times higher than normal.
Japan's nuclear agency later said there is no data suggesting the core has been breached.
Chief cabinet secretary Yukio Edano says a rigorous inquiry is under way to establish the cause of the leak.
Safety measures for people living near the plant have been revised, with the government asking people still within 20-30km of it to leave voluntarily.
A magnitude 8.9 earthquake and tsunami devastated north-east Japan on 11 March, killing more than 10,000 people. More than 17,440 people are missing.