Media playback is unsupported on your device Media caption Admiral Paul Zukunft, US Coast Guard: "There is no new oil coming up at the source"

BP is being allowed to keep the cap on its damaged well shut for another 24 hours, says the US official in charge of the Gulf of Mexico oil clean-up.

In a letter to BP, Adm Thad Allen expressed concern about a "detected seep" on the sea floor near the well and other "undetermined anomalies".

The firm has pledged to monitor the sea floor for signs of a new leak closely.

On Monday, Coast Guard Adm Paul Zukunft said: "We've had no indication of oil being released at the site."

BP has noted that seepage can be a natural phenomenon.

The fears over seepage coincided with a bad day for the oil giant in the stock markets. BP shares closed down more than 4.5% in London and plunged sharply during early trading in New York.

After installing a new cap, BP has been doing pressure tests to see if the well can be kept shut.

Adm Allen has said that if methane was found to be seeping from the sea floor, this might mean oil was also leaking.

He has ordered BP to submit a plan to reopen the well in case oil is still leaking. This would allow the crude to be piped to the surface.

But BP says it would take three days to start this process.

If the cap is reopened, the daily leakage of tens of thousands of barrels of oil could resume.

In a statement released on Monday, Adm Allen said a federal science team had held a conference call with BP representatives on Sunday night.

He said the scientists had received the answers they wanted about how BP was monitoring the seabed in case of any new leaks.

The well began leaking oil into the Gulf after BP's Deepwater Horizon oil rig exploded on 20 April, killing 11 workers.

BP had hoped the cap could stay in place until relief wells stopped the leak for good.

I direct you to provide me a written procedure for opening the choke valve as quickly as possible Adm Thad Allen, Letter to BP

But with pressure readings from within the well lower than expected, scientists had raised concerns that oil could be leaking into the surrounding undersea bedrock.

And in his letter to BP chief managing director Bob Dudley, Adm Allen said: "Given the current observations... including the detected seep a distance from the well and undetermined anomalies at the well head, monitoring of the seabed is of paramount importance...

"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 wellhead be confirmed."

The flow of oil from the well was shut off by the cap on Thursday and testing has continued since then.

'Worst disaster'

The Gulf of Mexico spill has been described as the worst environmental disaster the US has seen.

The spill has affected hundreds of miles of Gulf coastline, with serious economic damage as tourists have avoided Gulf Coast beaches and fishing grounds have remained closed.

BP has put the costs of dealing with the disaster at more than $3.95bn (£2.6bn).

It has already paid out more than $200m in compensation.

British Prime Minister David Cameron is to meet US President Barack Obama in Washington on Tuesday, and BP is expected to be a key topic of discussion.