Shares in BP were under pressure Thursday after the US government filed a civil lawsuit against it and eight other companies over the Deepwater Horizon oil rig explosion and the subsequent oil spill in the Gulf of Mexico.The complaint, which has been filed at a District Court in New Orleans, names eight other defendants including the insurer Lloyds of London, rig owner Transocean and oil exploration group Anadarko. The US is seeking unlimited damages.Halliburton, which supplied cement to plug the well, is not named in the lawsuit.Fines could exceed $21bn (£13.4bn), on top of the $30bn BP has already paid in compensation, if it is accused of gross negligence and this is proven. Damages are being sought under the Clean Water Act, which prohibits the unauthorised discharge of oil into US waters, and allows the US to seek damages of $1,100 for each barrel of oil spilled. "We intend to prove that these defendants are responsible for government removal costs, economic losses and environmental damages without limitation," said US Attorney-General Eric Holder."Even though the spill has been contained, the department's focus on investigating this disaster and preventing future devastation has not wavered."Assistant attorney-general Tony West said: "We have not asked for damages at this point because it's going to take years to fully quantify what the damages are."BP responded to the filing of the lawsuit, saying: "The filing is solely a statement of the government's allegations and does not in any manner constitute any finding of liability or any judicial finding that the allegations have merit."