Japanese trading house Mitsui & Co <8031.T> said BP Plc has sent it a $480 million bill seeking help in the clean-up of the disastrous Gulf of Mexico oil spill, but it has not decided if it will shoulder any costs.

Mitsui, which holds a 10-percent stake in the ruptured oil well through its unit Mitsui Oil Exploration, said it was carefully studying the bill and expected the embattled oil giant to seek more funds to help deal with the spill.

Mitsui and Canada's Anadarko Petroleum Corp , which has a 25-percent stake, had said last month they would not set aside money for the cleanup, arguing it was the responsibility of majority owner BP .

At the urging of the White House, BP set up a $20 billion fund to pay for future liability claims and other costs related to the spill, which killed 11 people and caused the worst environmental disaster in U.S. history.

Even if Mitsui is not required to help pay directly for the clean-up, it faces the risk of lawsuits and compensation claims in the United States, the prospect of which has weighed on its stock price in the past few months.

The $480 (million) as billed thus far is not necessarily representative because it doesn't include a lot of the lawsuits that will come down the pipe and that this could be a multi-tier clean-up, said Penn Bowers, an analyst at CLSA Asia-Pacific Markets.

Mitsui said its first-quarter net profit jumped 79 percent to 102.5 billion yen ($1.2 billion) even after it wrote down its stake in the well. It stuck by its forecast for profit to double to 320 billion yen profit forecast in the full year to March.

We expect to receive more bills from BP, Mitsui Chief Financial Officer Junichi Matsumoto said at its earnings news conference.

It's difficult to estimate at this moment the oil spill's total impact to our earnings. It's also difficult to tell when we can finish conducting such a calculation, he said, adding that the company was facing a number of lawsuits.

Shares of Mitsui & Co closed up 4.4 percent at 1,158 yen, outperforming a 1.3 percent rise in the benchmark Nikkei 225 <.N225>. The shares have slid 24 percent since April 20 when the explosion of BP's Deepwater Horizon drilling rig which later sank, unleashed a massive oil flow into the sea.

The full extent of the disaster is still slowly emerging.

U.S. government scientists on Monday refined estimates of how much oil had flowed into the Gulf from the well.

Investors will scrutinize these figures closely as BP's final costs may be tied to how much oil is estimated to have flowed into the Gulf from the spill.

(Additional reporting by Abhinav Sharma in Bangalore and by Benjamin Shatil in Tokyo; Editing by Edwina Gibbs and Valerie Lee)