Radioactive water is leaking into the sea from a 20-centimetre (8-inch) crack in a containment¹ pit at Japan's quake-hit Fukushima nuclear plant, its operator Tepco has said.

东京电力公司称，福岛核电站一座冷却池破裂，放射性污水透过一条20厘米长的裂缝渗透到海水之中。

The crack under reactor² 2 may be the source of recent radiation in coastal³ waters, Tepco officials said.

Tepco is preparing to pour concrete into the pit to try to stop the leak.

Japanese PM Naoto Kan has been visiting the area of north-eastern Japan worst affected⁴ by last month's tsunami⁵.

Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency, told a news conference Tepco was planning to pour concrete into the pit to seal the crack.

"With radiation levels rising in the seawater near the plant, we have been trying to confirm the reason why, and in that context, this could be one source," Mr Nishiyama said.

"There could be other similar cracks in the area, and we must find them as quickly as possible."

Tepco (Tokyo Electric Power Co) had earlier said it suspected radioactive material was escaping continuously from the plant, but they had not until now found the leak.

Measurements showed the air above the radioactive water in the pit contained 1,000 millisieverts of radioactivity.

Finding the source of the leak is a positive development, the BBC's Rachel Harvey in Tokyo says.

The challenge now though is to seal the opening, check there are no more elsewhere and remove the contaminated liquid, she adds.

On Friday, US Energy Secretary Steven Chu said about 70% of one reactor core was severely⁶ damaged as was about 30% of another. He said those figures were estimates because high radiation levels prevented workers from getting a close look at the damaged units.

Officials at Tepco and the nuclear safety agency would not confirm that assessment⁷, AP said.