Gulf spill: BP prepares for key oil-well test

Advertisement Adm Thad Allen: "When BP is ready, we will start to increase the pressure in the capping stack" Oil firm BP is preparing to test the strength of a cap it has placed on the leaking well in the Gulf of Mexico, in its latest bid to stop the flow.

BP began closing valves on Wednesday, but a leak was detected and testing will not begin until it has been fixed.

The government earlier delayed the whole procedure to allow experts to assess BP's plans, amid fears that a failure could make the situation worse.

An explosion on the Deepwater Horizon rig in April caused the leak.

"There is a tremendous sense of urgency," said Adm Thad Allen, the US incident commander. But he warned that nobody wanted to make "an irreversible mistake".

BP hopes the series of valves will be capable of holding back all of the oil from the well.

Once the leak in the choke valve has been fixed, experts will begin to test the integrity of the new capping device to check whether oil will leak out of other areas if valves on the blow-out preventer and new cap are closed.

The experts will monitor seismic, acoustic and temperature changes in the well and the surrounding area, as well as the pressure of the oil in the well.

High pressure will mean the oil has been contained inside the wellhead; low pressure could indicate that oil is leaking elsewhere in the well.

If the pressure proved to be low, BP officials said that the valves would not be kept shut.

If the valves are not able to hold back the flow, BP said it would still be able to channel some of the oil to container ships on the surface.

The White House said in a statement that the tests would provide "valuable data" about the condition of the well, and also a better understanding of how the well could be closed down during hurricanes.

Meanwhile, four senators from New York and New Jersey are pressing the US state department to look into allegations that BP lobbied for the release of Lockerbie bomber Abdelbaset al-Megrahi while attempting to finalise an oil deal with Libya.

Megrahi, who has terminal prostate cancer, was freed by Scottish Justice Secretary Kenny MacAskill on compassionate grounds in August 2009 after serving eight years.

The 1988 bombing of Pan Am flight 103 killed 270 people, mostly American.

BP has denied specifically lobbying for Megrahi's release, saying it was a matter for the UK and Libyan governments.