BP oil company has begun a crucial test on a leaking oil well in the Gulf of Mexico to prepare for an effort to plug the leak.U.S. officials approved the test late Wednesday.

BP officials say the test involves slowly choking the flow of oil from the undersea well, in an effort to measure the pressure of oil and natural gas escaping the well.

The test aims to determine if there is any damage in the well pipe, which runs nearly four kilometers below the sea floor in the Gulf of Mexico.

Coast Guard Admiral Thad Allen, who is overseeing the spill response, said he approved the test to help prepare for an effort to plug the leaking well. He said experts hope the test yields a high pressure reading, which will suggest there is no damage to the well pipe.

"If the pressure readings stay low, that will tell us the oil is probably going somewhere else. And we need to consider the fact we may have a breach in the well bore or in one of the casings," he said.

Admiral Allen said officials may end the test after a few hours if the pressure readings remain low. If not, it may continue for as long as 48 hours.

In either case, he said officials are likely to go ahead with an effort to kill the well, by pumping heavy mud into the well pipe to counter the flow of oil and natural gas. The pressure readings, however, will help crews know how much mud will be needed to fill the well bore and stop the leak.

"You can still kill the well, you would have to use a lot more mud. So your planning and some of the technical calculations you make vary greatly. So there is a direct relationship from the information gained in this well integrity test that will help us at killing the well."

Officials say they may try to kill the well as early as mid-August, when a relief well is expected to be finished. In the meantime, BP hopes to continue expanding its effort to siphon oil from the leaking well into surface ships to minimize damage to the Gulf of Mexico ecosystem.