The contaminated water emitted radiation 100,000 times the normal amount in a recent reading, the plant's operator Tokyo Electric Power Co. said.

"The radiation seems to have come from fuel rods that temporarily melted down and came in contact with the water used to cool the reactor," Edano told reporters.

The levels of radioactive iodine in the sea near reactors 5 and 6 has risen to 1,150 times higher than normal, the country's Nuclear and Industrial Safety Agency said.

Emergency workers are continuing efforts to determine the exact source of the radioactive leak, and are also spraying water on the reactors to try to prevent a meltdown.

The crisis was prompted by the March 11 earthquake and tsunami that killed more than 10,000 people in northeastern Japan.

A strong aftershock shook the region early on Monday and triggered a brief tsunami alert. There have been no reports of damage or injuries from the latest earthquake.