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The new containment capping stack is pictured Sunday in this image captured from a BP live video feed in the Gulf of Mexico. ((BP/Reuters))

Scientists are concerned about a seep and possible methane near BP's capped oil well in the Gulf of Mexico, a federal official said Sunday.

Both could be signs of leaks in the well that has now been plugged for three days.

The official spoke to The Associated Press on condition of anonymity because an announcement about the next steps had not yet been made.

The official, who is familiar with the spill oversight, would not say what is seeping near the well. The official said BP is not complying with the government's demand for more monitoring.

Thad Allen, the retired U.S. Coast Guard admiral in charge of the administration's spill response, demanded BP provide results of further testing of the seabed by Sunday at 9 p.m. ET.

"When seeps are detected, you are directed to marshal resources, quickly investigate, and report findings to the government in no more than four hours," Allen said in a letter to BP managing director Bob Dudley.

"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."

Earlier, BP spokesman Mark Salt declined to comment on the allegation, but said "we continue to work very closely with all government scientists on this."

Contradiction in plans

BP said Sunday it wants to keep using its giant stopper to block oil from reaching the Gulf until it can plug the blown-out well permanently.

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

But that differs from the plan the federal government laid out a day earlier.

Allen, the government's point man for the disaster, said Saturday that after the monitoring period ends, the cap will be hooked up to nearly 1.6 kilometres of pipes stretching to ships on the surface that will collect the oil.

But that would mean oil would have to be released back into the Gulf for three days to release pressure from the well, Suttles said. The oil giant hopes to instead keep the oil shut in until its permanent plug measure is completed, although Suttles said BP was taking it day by day.

Allen will make the ultimate decision. He insisted Sunday that "nothing has changed" since Saturday, when he said oil would eventually be piped to surface ships.

Trial period

Both Allen and BP have said they don't know how long the trial period during which they are monitoring pressure — initially set to end Saturday and extended to Sunday afternoon — will continue.

Allen decided to extend testing of the cap again, the unnamed official said. That means the oil will stay in the well for now as scientists continue to run tests and monitor pressure readings. The official didn't say how long that would take.

Unimpeded, the well spewed as much as 9.5 million litres a day, according to the government's worst-case estimates. It's possible the oil has been depleted, and that's why pressure readings from the cap have been lower than anticipated, BP has said.

Scientists still are not sure whether the shut-in is causing oil to leak into the bedrock surrounding the well, which could make the seabed unstable. That's why pumping the oil up to four ships on the surface and containing it there may be a safer option.

Relief wells

But to do that, millions of litres of oil could spew into the water when the cap is initially reopened, an image both BP and the federal government would like to avoid.

BP is drilling two relief wells, one of them as a backup. The company said work on the first one was far enough along that officials expect to reach the broken well's casing, or pipes, deep underground by late this month. Then the job of jamming it with mud and cement could take "a number of days through a few weeks."

The cap, which on Thursday stopped the crude for the first time since the April 20 explosion unleashed the spill, lets BP shut in the oil, which would be important if a hurricane were to hit the Gulf and force ships to leave the area.

Somewhere between 356 million and 697 million litres have spilled into the Gulf, according to government estimates.

With files from The Associated Press