Shares of BP plunged on Tuesday as investors worried about its liability for the costs of its massive oil leak in the Gulf of Mexico and White House promises of tougher industry oversight.

The company's shares plunged during afternoon trading on the London Stock Exchange, began to recover somewhat late in the morning in New York, but fell again over the noon hour as U.S. President Barack Obama promised a "full and vigorous" accounting of what caused the spill.

The shares ended trading in New York at $36.52, down $6.43, or 15 per cent.

Floating living quarters for oil spill cleanup workers, known as 'flotels,' are docked at Grand Isle, La., on Monday. BP said Tuesday its spill-related costs had reached close to $1 billion US. (Gerald Herbert/Associated Press)

Obama made his comments as political pressure grows on the White House amid criticism that it has not taken a strong enough role in combating the spill.

The remarks followed his first meeting with the co-chairs of an independent commission investigating the catastrophe.

Obama met with Bob Graham, a former Florida governor and U.S. senator, and William K. Reilly, a former head of the Environmental Protection Agency.

Obama spoke in the Rose Garden after the group's first meeting and pledged that necessary changes will be made. The president said that if current laws were insufficient, they'd be changed.

He said that if government oversight isn't tough enough, that would change, too. Obama also said if laws were broken, those who were responsible would be prosecuted.

U.S. President Barack Obama, accompanied by BP Oil Spill Commission co-chairs, former Florida senator Bob Graham, left, and former EPA administrator William Reilly, speaks in the Rose Garden of the White House on Tuesday. (Charles Dharapak/Associated Press)

Obama said the leaders of the commission have his support to follow the facts wherever they lead.

BP's shares dropped earlier in the day to their lowest level in more than a year as the British company revealed that costs for the spill have reached $990 million US.

Shareholders in the company have collectively lost $63 billion and shares have lost more than one-third of their value since the explosion at the Deepwater Horizon rig six weeks ago.

"This situation has now gone far beyond concerns of BP's chief executive Tony Hayward being fired, or shareholder dividend payouts being cut — it's got the real smell of death," said Dougie Youngson, oil analyst at Arbuthnot.

BP could be takeover target

"Given the collapse in the share price and the potential for it to fall further, we expect that it could become a takeover target, particularly if its operating position in the U.S. becomes untenable," he added.

BP said the costs so far relate to spill containment efforts, relief well drilling, grants to Gulf states, claims already paid and federal costs.

BP said it has received 30,000 claims and made more than 15,000 payments, totalling $40 million, and added it is too early to quantify other costs and liabilities.

Tuesday's trading session was the first response to the latest developments because the London market was closed on Monday for a public holiday.

The spill, which has dumped between 68 and 151 million litres of oil into the Gulf, according to U.S. government estimates, is the biggest in U.S. history.

With files from The Associated Press