BP's broken well is leaking oil and gas again for the first time since the company capped it last week.
The Obama administration's spill chief, however, said it was no cause for alarm and the stopper has been left in place for now.
Ever since the cap was used to bottle up the oil last week, engineers have been watching underwater cameras and monitoring pressure and seismic readings to see whether the well would hold or spring a new leak, perhaps one that could rupture the sea floor and make the disaster even worse.
Small amounts of oil and gas started coming from the cap late on Sunday, but "we do not believe it is consequential at this time", retired US Coast Guard Admiral Thad Allen, the government's spokesman for the crisis, said.
Seepage from the sea floor was also detected over the weekend less than two miles away, but Admiral Allen said it probably had nothing to do with the well. Oil and gas are known to ooze naturally from fissures in the bottom of the Gulf of Mexico.
At a briefing in Washington, Admiral Allen said BP could keep the cap closed for at least another 24 hours, as long as the company remained alert for leaks.
BP and the US government had been at odds over the company's desire to simply leave the cap in place and employ it like a giant cork in a bottle until a relief well being drilled deep underground can be used to plug up the well permanently.
Admiral Allen initially said his preference was to pipe oil through the cap to tankers on the surface to reduce the slight chance that the build-up of pressure inside the well would cause a new blowout. That plan would require releasing millions more gallons of oil into the ocean for a few days during the transition - a spectacle BP apparently wants to avoid.
But on Monday he budged a bit, saying unless larger problems developed, he was not inclined to open the cap. Somewhere between 94 million and 184 million gallons have gushed into the Gulf over the past three months in one of America's worst environmental crises.
BP said the cost of dealing with the spill has now reached nearly £2.6 billion. The company said it has made payments totalling £136 million to settle claims for damages. Almost 116,000 claims have been submitted and more than 67,500 payments have been made.