UK oil giant BP has further cut crude production at its Prudhoe Bay field in Alaska - the biggest in the US - because of a technical problem.
BP has to deal with investor concerns and environmental issues
BP said it had cut oil output by 90,000 barrels per day (bpd), lowering production to 110,000 bpd, or about a quarter of its normal level of pumping.
The company said the problem was caused by the failure of a gas compressor and would take several days to fix.
Earlier this month, BP closed half the field because of pipeline corrosion.
The further trimming of production at the Prudhoe Bay field was one of the factors behind a firming of global oil prices, traders said.
In London, a barrel of benchmark Brent crude gained 28 cents to $78.30. New York light crude gained 20 cents to $71.96 a barrel.
BP said it was looking for alternative routes through which to funnel its oil supplies.
Earlier this month, BP announced that it was cutting production at Prudhoe Bay to replace 16 miles of corroded pipeline at a cost of about $170m (£89m).
BP has been accused of misleading regulators over the condition of its pipelines, although it has denied all the allegations.
It had previously considered a full closure of the field.
BP's corrosion problems in Alaska are now being investigated by both the Alaskan attorney general, David Marquez, and the US Environmental Protection Agency.
The company's partners in the Prudhoe Bay field are ConocoPhillips and Exxon Mobil.