Whether to extend a test that has halted the oil leak in the Gulf of Mexico for three days is still pending, as engineers have detected seepage on the ocean floor near the well.

The test, which involves multiple pressure readings in the well, is designed to check the structural integrity of the blown-out well and to see whether it is structurally sound with no leaks in the seabed.

The U.S. government on Sunday ordered oil company BP to report on a "detected seep" near the well.

"Given the current observations from the test, including the detected seep a distance from the well and undetermined anomalies at the well head, monitoring of the seabed is of paramount importance during the test period," Coast Guard Admiral Thad Allen said in a letter to BP chief managing director Bob Dudley.

"I direct you to provide me with 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."

BP said earlier it hoped the well could remain sealed until a relief well permanently stops the flow with heavy drilling mud and cement -- a fix scheduled for mid-August.

But Allen was cautious and suggested that keeping the cap on was not a done deal.

"The ongoing well integrity test will continue until 4 p.m. (2000 GMT) today, with the potential for additional extensions in 24-hour increments," Allen said in a statement on Sunday.

Allen will make a final decision on the next step. A BP official said the testing could stop if Allen does not get the analysis of the seep.

"Ultimately, we must insure no irreversible damage is done which could cause uncontrolled leakage from numerous points on the sea floor," he said.

BP's so-called "well integrity test" began Thursday after two days of delays, first as government scientists scrutinized testing procedures and then as BP replaced a leaking piece of equipment known as a choke line.

The company said oil stopped leaking Thursday afternoon after engineers gradually dialed down the amount of oil flowing through the last of three valves on the cap.

It is the first time that BP completely stopped the oil leak since the BP-leased Deepwater Horizon drilling rig exploded and sank off the Louisiana coast in late April, killing 11 workers and unleashing the worst oil spill in U.S. waters.

Once the test is complete, BP will immediately resume siphoning crude oil from the well to the two oil-capture vessels on the surface.