The break in the pipeline connecting the California cities of Concord and Sacramento occurred on Tuesday night, Coast Guard spokesman Glynn Smith said. He added that between 300 to 600 acres of marsh land had been affected near Suisan Bay north of San Francisco.

Officials said they did not know how much fuel had leaked and it was not immediately clear why news of the spill did not become publicly known until Thursday.

"The original spill began approximately 7 p.m. PDT (10 p.m. EDT) on Tuesday and the computerized system that monitors the (Kinder Morgan line) detected a problem in the pipeline," Smith said. "The valves on either side of the breach were closed and Kinder Morgan sent out a crew."

Kinder Morgan Partners, controlled by Houston-based pipeline operator Kinder Morgan Inc., is the nation's largest pipeline master limited partnership.

Smith said that hot sun had helped evaporate much of the leaked diesel fuel since the spill.

"Yesterday, you could see a sheen on the water," the Coast Guard official said. "The folks coming back from the scene today are all telling me that you can't see anything, it's a hot day, a sunny day, and diesel is evaporating."

Jerry Engelhardt, a spokesman for Houston-based Kinder Morgan, said the 14-inch (35.5 cm) diameter pipeline, which is buried under Suisun Bay adjacent to the San Francisco-Sacramento railroad line, was shut down after the problem was detected.

He added that the source of the leak in the welded steel pipe was discovered on Wednesday in a marsh area. He said he did not know how much fuel was spilled.

The pipeline carries diesel and jet fuels and gasoline from refineries in the San Francisco Bay area to Sacramento and points east and north.