The US government will allow BP to keep the cap on its damaged Gulf of Mexico well closed for another day, despite the detection of seepage and a possible methane gas leak in the seabed.

The company promised to watch closely for signs of new leaks underground after a warning from Thad Allen, who is in charge of the US government's response to the disaster.

Allen said this morning BP had given government scientists the answers they needed about how it was monitoring the seabed around the mile-deep well, which has stopped gushing oil into the water since the experimental cap was closed on Thursday.

US scientists grilled BP engineers last night after Allen wrote a letter to the BP managing director, Bob Dudley, asking for written details about how the valve would be opened if necessary.

The letter 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 $3.95bn (£2.6bn), and has already paid out $207m 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.