"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 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."
Hydrocarbons occur naturally in crude oil, and their detection could mean that oil is seeping out from the well area. The well began gushing oil after BP's Deepwater Horizon oil rig exploded on April 20 and capsized two days later, killing 11 workers.
Transducers on BP's latest containment cap have given steadily increasing high-pressure readings since tests began on Thursday on the well bore, which stretches down four kilometres below the seabed.
Allen, the government's pointman on the worst oil spill in US history, said BP must maintain coordination with government monitors and report in no more than four hours when seeps are detected.
"As a continued condition of the test, you are required to provide as a top priority access and coordination for the monitoring systems, which include seismic and sonar surface ships and subsea ROV and acoustic systems," he wrote.
"When seeps are detected, you are directed to marshal resources, quickly investigate, and report findings to the government in no more than four hours."
Allen demanded that BP "provide me a written update within 24 hours of your intentions going forward. I remain concerned that all potential options to eliminate the discharge of oil be pursued with utmost speed until I can be assured that no additional oil will spill from the Macondo Well."
Allen earlier told reporters that testing would continue on the well in order to guard against seabed cracks.
"Ultimately, we must insure no irreversible damage is done which could cause uncontrolled leakage from numerous points on the sea floor," he said. (