The US Justice Department reached a $1.4bn settlement with Transocean Ltd., the owner of the drilling rig that sank after an explosion killed 11 workers and spawned the massive 2010 oil spill in the Gulf of Mexico.

The proposed settlement announced on Thursday resolves the department's civil and criminal investigation of the Switzerland-based Transocean's role in the Deepwater Horizon rig disaster, the worst environmental disaster in US history.

In pleading guilty, the department said, Transocean had admitted its rig crew was "negligent" in securing the well.

It requires the company to pay $1bn in civil penalties and $400m in criminal penalties and plead guilty to a misdemeanour charge of violating the Clean Water Act, according to a court filing.

The deal is still subject to a federal judge's approval. It calls for Transocean to implement a series of operational safety and emergency response improvements on its rigs.

"This resolution of criminal allegations and civil claims against Transocean brings us one significant step closer to justice for the human, environmental and economic devastation wrought by the Deepwater Horizon disaster,'' Attorney General Eric Holder said in a statement.

Transocean described the settlement as a positive development.

"This is a positive step forward, but it is also a time to reflect on the 11 men who lost their lives aboard the Deepwater Horizon," the company said in a statement. "Their families continue to be in the thoughts and prayers of all of us at Transocean.''

Much of $1.4bn will fund environmental restoration projects and spill-prevention research and training. The company has two years to pay the $1bn civil penalty.

BP Settlement

In a separate settlement last November, British oil giant BP PLC, which leased the rig from Transocean, agreed to pay a record $4.5bn in penalties and plead guilty to manslaughter and other criminal charges related to the spill.

The deal with BP does not resolve the federal government's civil claims against the oil company.

BP estimates it will pay about $7.8bn to resolve these claims, but the settlement isn't capped.

The Deepwater Horizon was drilling in water a mile deep southeast of the Louisiana coast when it exploded on the night of April 20, 2010.

The Justice Department says Transocean crew members on the rig, acting at the direction of BP supervisors, failed to fully investigate clear signs that the well was not secure and that oil and gas were flowing into the well.

The rig burned for about 36 hours before sinking.

As engineers made repeated attempts to halt the flow of oil from BP's burst well over several weeks, millions of gallons of crude flowed out.

Marshes, beaches and fishing grounds across the northern Gulf were fouled by the oil.