Washington, Aug 14 (DPA): Oil giant BP must complete work on a relief well in order to be certain that the worst oil spill in US history has been stopped for good, government officials said Friday.

But new tests taken Thursday raised fresh warnings and left engineers uncertain how to proceed with drilling the relief well, currently idle about 10 metres away from the source of a months-long ecological disaster in the Gulf of Mexico.

While a cementing procedure earlier this month may have already sealed off the entire ruptured well, Coast Guard Admiral Thad Allen made clear that only the completion of the relief drilling could make certain that the gusher had been permanently stopped.

"Everybody is agreed, we need to proceed with the relief well. The question is how to do that," said Allen, who is coordinating the government's response to the disaster.

The relief well had long been viewed as the last step in permanently plugging the ruptured oil well, which has leaked about 4.9 million barrels of oil into the Gulf since the April 20 explosion of the Deepwater Horizon drilling rig.

But Allen Thursday said the relief well may no longer be necessary. The so-called "static kill," an earlier injection of cement from the top of the ruptured well pipe down into the reservoir below, may have been enough.

Allen said Friday a new round of pressure testing of the well left engineers without the proof they needed to be sure the static kill had ended the long-running saga.

"The relief well will be finished. We will kill the well," Allen said.

BP and government officials were meeting through Friday to evaluate the pressure readings and iron out their concerns. Once the drilling is restarted, it will take about 96 hours to be completed, Allen said.