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

The containment cap on top of a blown-out oil well in the Gulf of Mexico will remain closed for an additional 24 hours despite concerns over nearby seepage and leaking from the wellhead, a U.S. official says.

Thad Allen, the retired U.S. Coast Guard admiral who is the U.S. administration's point man on the disaster, said Monday he had authorized BP to continue the integrity test on the custom-built cap for another day on the condition that the company would continue to monitor for "any signs that this test could worsen the overall situation."

Late Sunday, Allen said a seep had been detected about 3.2 kilometres from the wellhead and in a sharply worded letter demanded that BP step up monitoring of the ocean floor.

By Monday afternoon Allen said this was not a major concern. "We do not believe it is associated with this particular well integrity test," he said.

However, Allen confirmed that a leak had been detected on the capping stack and added that bubbles had also been observed around the blowout preventer.

Concern over leak

In a statement released early Monday, the company said "extensive monitoring activities" are being carried out around the well site.

The concern all along — since pressure readings on the cap weren't as high as expected — has been that there is a leak elsewhere in the well bore, meaning the cap may have to be reopened to prevent the environmental disaster from becoming even worse and harder to fix.

An underground leak could let oil and gas escape uncontrolled through bedrock and mud.

BP said Monday that pressure inside the well continues to rise slowly.

The integrity tests began after the company closed the valves on the temporary cap Thursday afternoon, stemming the flow of oil into the Gulf of Mexico for the first time since the rig exploded on April 20.

In the letter sent Sunday, Allen also stressed the importance of the relief wells, which are viewed as a more permanent solution to stopping the leak, saying, "the primary method of securing the source is the relief well and this effort takes precedence."

The company said Monday that work on the first relief well continues, while work on the second relief well has been temporarily halted so as not to interfere with tests conducted on the first well.

BP said Monday that "although uncertainty remains," the current estimate is that the first relief well will be completed in the first half of August.

Costs approach $4 billion US

BP said Monday that the cost of dealing with the oil spill has now reached nearly $4 billion US.

The company said it has made payments totalling $207 million to settle individual claims for damages from the spill along the southern coast of the United States.

To date, almost 116,000 claims have been submitted and more than 67,500 payments have been made, totalling $207 million.

With files from The Associated Press