The Tokyo Electric Power Co. (TEPCO) had earlier estimated the height of the wave at 10 metres at its Fukushima No. 1 plant, some 250 kilometres (155 miles) northeast of Tokyo.

A massive 9.0-magnitude quake on March 11 triggered colossal waves along the country's Pacific coast, crippling the plant's cooling systems and prompting emergency operations to prevent fuel rods from melting and spewing radioactive material.

"Now we estimate the height at more than 14 metres. We have found traces of the tsunami at such elevations," TEPCO spokesman Naoki Tsunoda said, adding that the wave was 14 metres high when it passed through the plant's parking area.

A tsunami can surge to an elevation higher than its height at the time when it hits shore, Japanese media noted.

The stricken plant's twin complex, some 10 kilometres (six miles) to the south, was also hit by the tsunami but received less extensive damage.

An unidentified subcontract worker at the Fukushima No. 2 plant told public broadcaster NHK that he evacuated to a hill immediately after the quake crushed an embankment and broke the arm of a crane at his plant.

"There was a backwash which left the seabed clearly seen some 200 metres offshore from the beach," he said.

"Then the tsunami approached all at once and surged onto the plant.

"The tsunami cleared high above the dyke and came rushing down to wash away one parked car after another. I was very scared," he said.

The plants were designed to withstand earthquakes of magnitudes up to 8.0 and tsunami waves of up to 5.7 metres at the No. 1 plant and 5.2 metres at the No. 2.

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