Oil producer BP Plc began shutting down on Sunday the biggest oilfield in the United States, Alaska's Prudhoe Bay, after discovering a small pipeline leak, cutting output by 400,000 barrels daily and sending oil prices up nearly 2 percent. BP, already part of a criminal probe into a much bigger Alaskan pipeline rupture in March, was unable to estimate when output might resume at the field, which accounts for about 8 percent of U.S. domestic crude oil production. The closure is the latest incident to hit BP's Alaskan operations, a cornerstone of its global upstream portfolio, and deals another blow to its U.S. image following a deadly refinery explosion last year and a trading scandal. "We regret that it is necessary to take this action and we apologize to the nation and the State of Alaska for the adverse impacts it will cause," newly appointed BP America Chairman and President Bob Malone said in a statement. Prudhoe Bay is operated by a BP-led group that includes oil majors ConocoPhillips and Exxon Mobil Corp., which itself was involved in Alaska's biggest oil spill when the 11 million-gallon Exxon Valdez tanker was grounded in 1989. BP gave no estimate on how long the field would be offline, but shutting down a major onshore oil facility -- a process that takes at least several days -- is usually a last resort.