It finally stopped the flow of 62,000 barrels of oil per day into the ocean after 90 days by placing a cap over the top.

The relief well is seen as the final hurdle, but BP raised questions over whether it is necessary.

• BP oil spill: full coverage

"It could be seen as opening up an old wound when surgery has already fixed the problem," one source said.

However, the US Incident Command said on Friday that the relief well was necessary to make sure the well is permanently killed.

The oil company has been conducting pressure tests to determine whether cement has entered the layer between the well casing and wall called the annulus –
providing a solid seal.

The presence of rising pressure determines the need to continue with a second kill from the bottom, according to Admiral Thad Allen, who leads the
Incident Command Centre.

Opening up the well through relief drilling raises a small risk that oil could flow again into the Gulf of Mexico if anything were to go wrong. Drilling on
the relief well began in early May, proceeding three miles from the surface
and two miles beneath the sea floor to within 30 to 50 feet of the exploded
well.

The technical developments came as the US state of Alabama said it was suing BP and its contractors Transocean and Halliburton for damage caused to the
Gulf state by the oil spill.

"We are making this claim because we believe that BP has inflicted catastrophic harm on the state," Troy King, Alabama's attorney general,
told Reuters.

The state is seeking punitive damages but it has not specified an amount.

It accuses BP of damaging Alabama's environment and economy through "negligent or wanton failure to adhere to recognised industry standards".

BP declined to comment on legal action.