The suit was announced as BP and US government officials decided to proceed with a relief well to ensure that the blown-out well remains plugged. 

Pressure tests

"Everybody is in agreement that we need to proceed with the relief well," retired Coast Guard Admiral Thad Allen, who is overseeing the effort to bring the spill under control, said. "The question is how to do it."

in depth

Allen had earlier raised the possibility that the relief well might not be necessary because the cement poured into the top of the welllast week might have permanently killed it, but after running pressure tests it was decided it was still needed.

The tests, which were completed late on Thursday, showed that the well is effectively sealed, with "no communication with the reservoir," Allen said. 

In video

US fishermen battle oil slick

But engineers are worried that increased pressure from the the "bottom kill" could damage the existing temporary cement plug and perhaps cause about 1,000 barrels of oil trapped in the well shaft to flow into the ocean.

After Allen gives the order to continue with the relief well, it will take about 96 hours to drill into the Macondo well shaft and perhaps days beyond that to complete the job, he said.

BP hired the Deepwater Horizon drilling rig, which sank into the Gulf of Mexico after a fire in April triggering the massive spill, from owner Transocean.

Halliburton provided the cement work for the well while Anadarko Petroleum Corp, which is also named in the suit, was a minority owner.

'Dangerous risks'
"It is believed the explosion on the Deepwater Horizon was a blowout relating to the cementing work," Alabama's suit said.

The spill has hurt industries such as fishing and tourism on the Gulf coast [Gallo/Getty]

It accuses BP and the other defendants of "negligent or wanton failure to adhere to recognised industry standards of care".
"The defendants' conduct in operating the Deepwater Horizon and oil well illustrates their scheme to maximise profits and ignore dangerous risks posed to human health and property," the suit said.

The spill, which saw an estimated 4.9 million barrels of oil leak into the water, has hurt industries including fishing and tourism and has affected other sectors such as housing.

People and businesses that have sustained losses can claim against a $20 billion BP compensation fund administered by Kenneth Feinberg, named by the White House as an independent overseer.

BP said on Thursday it had paid out $347m in claims since May 3, after receiving 148,000 claims. It said it has yet to deny a single claim, although about 40,000 claims were still outstanding or awaiting adjustment.