Tokyo, April 3 (IANS) Japan's attempt to cement a cracked concrete pit at a damaged nuclear reactor has failed as the toll in the earthquake that spawned a giant tsunami crossed 12,000.

The nuclear complex at Fukushima was damaged March 11 in a magnitude-9 earthquake and ensuing tsunami. Its cooling system was damaged, causing the reactors to overheat.

On Saturday, the crippled nuclear plant was found to have been discharging highly radioactive water into the Pacific Ocean, Xinhua reported Sunday.

Fukushima nuclear plant's operator Tokyo Electric Power Co. (TEPCO) said it had found a crack in the pit at its nuclear reactor 2, which they believed was the source of the contaminated water.

Soon after the discovery, the plant's operator started filling the pit with cement to seal the crack and prevent more contaminated water from seeping into the ocean.

However, after pouring concrete into the pit, workers still found heavy doses of radiation that was over 4,000 times the legal limit.

After the failed attempt, TEPCO's engineers were struggling to devise new tactics such as using water-absorbent polymer to prevent more leakage.

According to nuclear safety officials, the crack in the nuclear plant was probably caused by the quake and could be the source of radioactivity found in coastal waters in recent days.

The toll in the quake that triggered a giant tsunami in Japan has surpassed 12,000 while nearly 15,500 were unaccounted for, the Kyodo news agency reported Sunday.

Over 165,000 people have been evacuated from the disaster zone and accommodated in temporary refuge centres across the country.

The Japanese government has said the damage from the disaster could total $310 billion.