ANCHORAGE, Alaska - BP has discovered a leak in an oil pipeline from one well at the giant Prudhoe Bay field, the third pipeline leak reported by the oil major over the past month on Alaska's North Slope.

BP said there was no appreciable impact to production from the event, which spilled a mixture of oil, produced water and natural gas at Prudhoe Bay, the largest U.S. oil field.

This was, by every indication, an extremely short-term event. The line broke, the safety valve immediately shut the well off, it was over, said BP Exploration (Alaska) Inc spokesman Steve Rinehart.

The amount of liquid spilled was unknown, state and BP officials said, adding it came from a 6-inch-diameter pipeline that carries product from a single well. The leak was discovered on Monday during a routine check, state environmental officials said.

Rinehart said the spilled material appeared to have been limited to the area right by the well, landing on top of snow on the gravel pad.

Weld Royal, spokeswoman for the Alaska Department of Environmental Conservation, said the spill appeared to have definitively affected about 12,000 square feet of gravel pad, along with another 5,000 square feet that had patches of spilled material. Officials were hoping Tuesday to delineate possible impacts to snow-covered tundra, Royal said.

Three weeks ago, a ruptured 18-inch-diameter line at the Lisburne field released about 46,000 gallons of liquids, a mixture of oil and produced water, according to state officials. That rupture, discovered on November 29, is believed to have been caused by pressure that built up when contents of the line froze into long ice plugs. That event remains under investigation, officials said.

BP is preparing to fix the broken pipeline, a plan that would require state approval, Royal said. BP has given DEC a conceptual plan for replacing the affected part of the pipeline. DEC is reviewing that conceptual plan, Royal said.

Another spill, discovered December 2, was from a pipeline inside a manifold building at a different Prudhoe Bay drill site. That spill released an estimated 7,170 gallons of produced water, according to state environmental officials.

BP is on probation following an Alaska pipeline spill in 2006, the largest ever recorded on the North Slope. That spill poured 212,252 gallons of crude oil onto the tundra. The company pleaded guilty to a single criminal violation of the Clean Water Act and paid $20 million in fines and restitution. It was also ordered to improve its pipeline maintenance.

Various agencies are investigating the circumstances of the Lisburne pipeline rupture to see whether there were any violations of BP's probationary terms, said Mary Frances Barnes, BP's federal probation officer.

A result of that probe will probably not be reached quickly because so many agencies are participating, Barnes said Monday. The more people you have involved, the longer it's going to take, she said.

BP is also the target of pending civil lawsuits filed by both the state and federal governments over the 2006 spill and a second spill that year that resulted in a partial shutdown of the Prudhoe Bay field.

The federal lawsuit seeks millions of dollars in fines for various environmental violations. The state lawsuit seeks hundreds of millions of dollars in compensation for what the state claims was royalty and tax revenue lost during Prudhoe's partial shutdown in 2006.

At the site of the Lisburne spill, which sent oil and produced water spraying over an area of about 8,400 square feet, cleanup is nearly completed, state and BP officials said.

Oil-covered snow has been hauled away and is being melted, with contents separated, according to the Alaska Department of Environmental Conservation. Workers using jackhammers and hand tools have removed contaminated snow and ice in spots inaccessible to machines, Rinehart said.

What can be done now is almost completely done, he said.

Crews will be at the site in the spring to check on potential damage to tundra and, if necessary, options for rehabilitation, according to the Department of Environmental Conservation.

(Editing by Christian Wiessner)