According to Reuters a 40-person crew was working around the clock with on Thursday to clean up the spill mainly with "shovels and brooms".

The spill was caused by a ruptured well line to the BP-operated Prudhoe Bay field with state data reportedly revealing North Slope production had dropped by about 10,000 barrels per day.

"Just how much liquid escaped from the line remains under investigation by BP and Alaska's Department of Environmental Conservation," the newswire reported on Friday.

"It remains unclear whether the leak, detected earlier this week, is connected to a decline in North Slope oil production. BP did not return emails seeking comment."

The leak was reportedly discovered during a route investigation on Monday afternoon with the line later depressurised to halt its flow.