US nuclear regulators published an update on California's troubled San Onofre power plant Thursday, sparking an expert warning that the problem is more serious than first thought.

A reactor at the nuclear power plant near San Diego was shut down in January after a radiation leak, although the Nuclear Regulatory Commission (NRC) said there was no danger to the public.

Investigations found unexpected erosion on tubes that carry radioactive water, and the entire plant was shut down, forcing Californian authorities to fire up alternative power generation facilities.

On Thursday, an update on the tube erosion, posted on an obscure part of the NRC's website, showed the situation had worsened.

"This reveals a far greater problem than has been previously disclosed, and raises serious questions about whether it is safe to restart either unit," said Daniel Hirsch, a nuclear expert at the University of California, Santa Cruz.

The new data shows that more than 3,400 steam generator tubes in the new steam generators at San Onofre have been found to be damaged -- about 1,800 in Unit 3 and 1,600 in Unit 2 -- he said.

"Edison had been talking about trying to get Unit 2 back on line at end of summer; now we know to do so they would have to run with a large number of damaged tubes," Hirsch told AFP.

A spokeswoman for operator Southern California Edison did not immediately respond to a request for comment on the new figures. The NRC did not comment on the figures, which were presented as tables.

San Onofre produces enough energy to power 1.4 million homes, according to SCE. California's only other nuclear reactor, at Diablo Canyon between Los Angeles and San Francisco, is run by Pacific Gas and Electric Company.