Officials are monitoring the clean-up of a leak of 400,000 gallons (1.5m litres) of radioactive water from a local nuclear power plant in Minnesota.

Xcel Energy, the utility company that runs the plant, said the spillage was "fully contained on-site and has not been detected beyond the facility".

State officials said there was no immediate public health risk.

The leak was first discovered in late November, but state officials did not notify the public until Thursday.

The water contains tritium, a common by-product of nuclear plant operations.

A naturally occurring radioactive isotope of hydrogen, tritium emits a weak form of beta radiation that does not travel very far in air and cannot penetrate human skin, according to the federal Nuclear Regulatory Commission (NRC).