WASHINGTON, July 16 (Reuters) - U.S. nuclear regulators say they are ready to send technical experts after a strong earthquake hit Japan on Monday, sparking a small radiation leak and fire at the world's biggest nuclear power plant.

Tokyo Electric Power Co. <9501.T> shut down three major generators at the Kashiwazaki-Kariwa nuclear plant after a powerful earthquake in Japan on Monday caused a brief fire in one of the units, company officials said.

"We're gathering information, and if any requests for assistance are made we are ready to respond under our existing agreement," said Scott Burnell, a spokesman for the U.S. Nuclear Regulatory Commission, which oversees the 104 operating U.S. civilian reactors.

TEPCO, Asia's biggest utility, said 1.5 litres (three pints) of water containing radioactive materials had leaked from a unit closed for maintenance at the Kashiwazaki-Kariwa plant.

Burnell did not comment on the state of the nuclear plant.

U.S. and Japanese nuclear officials have a standing agreement to share technical assistance in the event of a nuclear incident -- which could include sharing of NRC staff expertise, Burnell said.

"If Japan makes any request of us we will work with them under the existing bilateral agreement," he said.

The contaminated water was released into the ocean and had had no effect on the environment, TEPCO said in a statement. The company had previously said there had been no radiation leaks at the plant, where reactors automatically shut down for checks.

At least seven people were killed and more than 800 people were injured by the quake in northwestern Japan, and buildings swayed as far away as Tokyo. Thousands were evacuated from their homes.

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