By Matthew Weaver, The Guardian

Concerns are mounting about the new cap over BP’s damaged Gulf of Mexico oil well after engineers detected seepage and a possible methane gas leak on the seabed.

Admiral Thad Allen, who is in charge of the US government’s response to the disaster, has written to BP demanding answers to "undetermined anomalies at the well head".

US scientists grilled BP engineers last night about how the site is being monitored. Allen gave BP permission to keep the containment cap closed for another 24 hours while investigations continue.

In the earlier letter to the BP managing director, Bob Dudley, he said: "When seeps are detected, you are directed to marshal resources, quickly investigate, and report findings to the government in no more than four hours. I direct you to provide me a written procedure for opening the choke valve as quickly as possible without damaging the well should hydrocarbon seepage near the well head be confirmed."

The apparent setback caused shares in the oil company to fall more than 5%, though by lunchtime losses had been pared to 2.5%. The company has put the costs of dealing with the disaster at over $3.95 billion, and has already paid out $207 million to 67,500 claimants who have lost money due to the spill.

A BP spokesman said that if a seep is confirmed from the well, the cap will be lifted and oil flowed to the surface. In a characteristically technical statement it said: "As directed by the National Incident Commander [Allen], extensive monitoring activities are being carried out around the well site. Information gathered during the test is being reviewed with the government agencies to determine next steps. Depending upon the results of the test and monitoring activities, these steps may include extending the well integrity test or returning to containment options."

Yesterday, officials said the cap was holding and continuing to prevent oil spewing into the gulf for the first time since the rig exploded in April, killing 11 workers.

They expressed hope that it could stem the leak until relief wells were in place to permanently shut off the flow of oil ‑ but the discovery of seepage could mean there are still leaks in the damaged well.

The plan had been for BP to pipe oil to the surface, which would ease pressure on the well but require up to three more days of oil spilling into the gulf.

Doug Suttles, BP’s chief operating officer, said yesterday: "No one associated with this whole activity … wants to see any more oil flow into the Gulf of Mexico. Right now we don’t have a target to return the well to flow."

The new potential blow comes as David Cameron travels to Washington later today for his first full-length bilateral meeting with the US president, Barack Obama, at which they are expected to discuss the BP oil crisis. The spill is the worst in US history, causing economic and environmental disaster in five states along the Gulf coast and threatening to sour Anglo-American relations.

(Photo: Petty Officer 3rd Class Patrick Kelley)

See also:

Gulf Oil Spill Reaches the Senate in a Shrimper’s Jar

Investigator Warned MMS in 2009 About Deepwater Gas Blowouts in Gulf of Mexico

Did Deepwater Methane Hydrates Cause the BP Gulf Explosion?