* U.S. concerned over seep detected near BP well
* BP aims to keep new cap closed as long as possible
* British PM Cameron visits Washington this week (Adds details, background, WSJ report)
HOUSTON, July 18 (Reuters) - Engineers monitoring BP Plc's damaged well in the Gulf of Mexico detected seepage on the ocean floor that could mean problems with the cap that has stopped oil from gushing into the water, the U.S. government's top oil spill official said on Sunday.
Earlier on Sunday, BP officials had expressed hope that the test of the cap which began Thursday could continue until a relief well can permanently seal the leak next month. Oil gushed from the deepsea Macondo well for nearly three months until the new cap was put in place last week.
But late on Sunday, the U.S. government released a letter to BP Chief Managing Director Bob Dudley from retired Coast Guard Admiral Thad Allen that referred to an unspecified type of seepage near the mile-deep (1.6 km-deep) well along with "undetermined anomalies at the well head."
"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," Allen wrote.
BP did not respond to requests for comment on Allen's letter.
The worst oil spill in U.S. history has caused an economic and environmental disaster in five states along the Gulf Coast, hurt President Barack Obama's approval ratings and complicated traditionally close ties with Britain.
Those concerns are sure to be discussed when British Prime Minister David Cameron meets Obama in Washington on Tuesday.
The plan had been for BP to resume siphoning the oil after the completion of the pressure tests on the well, which extends 2.5 miles (4 km) under the seabed, to judge if it is able to withstand the process to seal the leak.
But Doug Suttles, BP's chief operating officer, said the company now hopes to keep the damaged well shut until the relief well is completed in August and the leak is sealed off with heavy drilling mud and cement.
"We're hopeful that if the encouraging signs continue that we'll be able to continue the integrity test all the way to the point that we get the well killed," he told reporters before Allen issued his statement. "Clearly we don't want to reanimate flow into the Gulf if we don't have to."
Suttles' statement could indicate diverging viewpoints between BP and the U.S. government on plans for the well integrity test. It prompted Allen -- who will ultimately make the final call on the test -- to issue a statement that "nothing had changed" in the joint plan going forward.
When BP choked off the flow a mile (1.6 km) under the water's surface with a new, tighter cap on Thursday, it was the first time oil has not spewed since an April 20 explosion on an offshore rig killed 11 workers and triggered the disaster.
U.S. authorities probing the spill are looking into why workers missed signs of an impending explosion and have drawn up a list of more than 20 anomalies in the crew's response to them, the Wall Street Journal reported.
Billy Nungesser, president of Plaquemines Parish in Louisiana, said the new cap was good news after a three-month losing battle to try to clean up oil hitting fragile marshlands as more lapped ashore.
"We're very optimistic," Nungesser told the "Fox News Sunday" program. "We see light at the end of the tunnel. It's a very long tunnel but today we're making progress."
In Boothville, Louisiana, where a sign on a roadside snack stand said "Thank you, Jesus, the well is capped," residents were happy but frustrated at the economic toll the spill is taking.
Obama -- under fire to push BP to plug the leak and clean up a spill that has fouled beaches and ravaged fishing, tourism and drilling industries -- welcomed the success of the new cap but said there was much work ahead on a permanent solution.
Beyond monitoring the meeting between Cameron and Obama, BP and its lobbyists are keeping a close eye on the U.S. congressional debate over an energy bill that could include reforms meant to prevent a repeat of Gulf of Mexico spill.
Lawmakers are considering a range of new rules that could require tougher safety regulations on offshore drilling or bar companies like BP from new offshore exploration leases.
The crisis took on a new twist over the weekend as the British government said there was no evidence of a connection between BP and last year's release of a Libyan man convicted of the 1988 airline bombing over Lockerbie, Scotland, that killed 270 people, most of them Americans.
The U.S. Senate Foreign Relations Committee has scheduled a July 29 hearing on possible ties between BP and the release of Abdel Basset al-Megrahi, a Libyan intelligence officer who was the only person convicted of the bombing of Pan Am Flight 103.
BP has said it lobbied the British government about slow progress in resolving a different prisoner transfer agreement with Libya in 2007 but was not involved in Megrahi's release. (Additional reporting by Paul Eckert in Washington, Eileen O'Grady in Houston, Alexandria Sage in Louisiana; Writing by Deborah Charles; Editing by John O'Callaghan and Jackie Frank)