Offshore drilling group Transocean, the owner of the Deepwater Horizon platform that sank off the southern US coast, on Thursday rejected any responsibility for the subsquent giant oil spill.

The US government filed a lawsuit on Wednesday against BP and eight other companies, including Transocean, for uncounted billions of dollars in damages for the worst oil spill in US history in the Gulf of Mexico last April.

Transocean said in a statement e-mailed to AFP that it was not liable for discharges from the well owned by the oil giant.

"No drilling contractor has ever been held liable for discharges from a well under the Oil Pollution Act of 1990," a spokesman for Transocean said.

"The responsibility for hydrocarbons discharged from a well lies solely with its owner and operator," he added.

"This fact is made clear by both the letter and spirit of the law, is reinforced in official contracts between BP and other parties -- including Transocean and the US Government -- and was reaffirmed by BP in public statements following the incident."

"Transocean is indemnified in this matter," he said.

The British petroleum giant is the principal owner and operator of the oil field.

US Attorney General Eric Holder said the complaint alleges that "violations of safety and operational regulations" caused the explosion on the Deepwater Horizon rig, which sent nearly five million barrels of oil gushing into the Gulf.

He said the US government intended to prove "that the defendants are therefore responsible under the Oil Pollution Act for government removal losses, economic losses, as well as environmental damages."

Transocean's share price fell by 3.4 percent to 67.4 Swiss francs by close of trading on Thursday, on a largely stable market.

Analysts at Swiss prvate bank Vontobel estimated that the total damages could reach about 5.0 billion dollars (3.8 billion euros), on the basis of US legal provisions for a 1,100 dollar fine per barrel of oil spilled.

Transocean is covered by civil liability insurance of up to 950 million dollars, they added.