Image caption The final bill BP faces is unknown

One of BP's partners in the deepwater well which exploded in April - leading to the oil spill in the Gulf of Mexico - has refused to pay its share of the costs so far.

Houston-based Anadarko, which owns 25% of the Macondo well, was asked to pay $272m (£180m) by BP.

But an Anadarko spokesman said it was going to withhold payment.

BP said it was "disappointed" that the firm had "failed to live up to their obligations".

"Anadarko's refusal to pay their share will in no way affect BP's commitment to stop the leak, clean up the spill, and pay all legitimate claims as quickly as possible", spokesman Mark Salt added.

He said another firm with a stake in the project, Mitsui Oil Exploration, had also not responded to a request to help pay for the spill.

BP's bill so far has been more than $3.1bn - and it has agreed to set up a $20bn fund to deal with compensation claims and clean-up costs.

But investors remain uncertain over the eventual financial impact of the oil disaster, with estimates from analysts putting the final bill in the tens of billions of dollars.

BP shares remain at around half their value before the disaster and there has been speculation that it may look to sell a stake of the business to outside investors.

'Warning signs'

Anadarko indicated last month that it felt it should not pay for the spill, with its chief executive Jim Hackett issuing a statement that blamed BP for "reckless decisions and actions".

He accused the oil giant of failing to "react to several critical warning signs" as it drilled below the sea floor.

On Thursday BP said its operation to drill a new relief well to stop the Deepwater Horizon leak was ahead of schedule.

The new cap BP intends to install should make a more secure seal over the wellhead.

The company also plans to connect a third containment ship to capture more oil.