BP Plc’s costs for the worst spill in U.S. history appeared set to rise as a partner in the out-of-control well laid the blame at BP’s feet and the new federal czar overseeing damage claims said BP would pay more if $20 billion was not enough.

The British oil company said it would not be distracted by a dispute with Anadarko Petroleum Corp. The owner of a quarter of the well gushing into the Gulf broke its near-silence on the spill to squarely pin blame — and financial responsibility — on BP.

“There appears to be gross negligence or willful misconduct,” Houston-based Anadarko Chairman and CEO Jim Hackett said in an interview that helped to drive his company’s shares up 2.2 percent in after-hours trading on the hopes it could avoid multi-billion-dollar liabilities.