NEW ORLEANS — A seep has been detected around BP’s collapsed oil well in the Gulf of Mexico, causing new concerns about how well a newly installed cap is working and prompting the government to demand that BP step up its monitoring of the seabed.

Retired Coast Guard Admiral Thad Allen announced the seep late on Sunday and made the demands in a letter to BP. He did not say what is coming from the seep.

Since pressure readings on the cap weren’t as high as expected, the concern is that there is a leak elsewhere in the wellbore, meaning the cap may have to be reopened to prevent the environmental disaster from becoming even worse and harder to fix.