Workmen install casing for the relief well on June 19 at the site of the Deepwater Horizon oil spill off the coast of Louisiana. (Peter Andrew Bosch/Associated Press)

Drilling on a relief well must continue until it intercepts BP's blown-out well in the Gulf of Mexico, the U.S. government said Friday.

BP had publicly mused that it might not need to finish the relief well because the mud and cement it had previously pumped in to plug the leak — the so-called "static kill" — appeared to have essentially killed the well.

However, retired Coast Guard Admiral Thad Allen, who's been directing the federal response to the crisis, said Friday that drilling on the relief well must proceed.

"The relief well will be finished," he said at a news conference. "We will kill the well."

He said results of a pressure test conducted Thursday fell short of guaranteeing there would be no further oil spilled.

Drilling almost finished

Drilling of the first of the relief wells could be finished in just a few days. It's believed to be no more than 15 metres away from intersecting the damaged well.

Once it hits its target, engineers will pump in mud and cement from below — a manoeuvre designed to permanently plug the well.

Drilling of the main relief well and a backup well was temporarily suspended earlier this week when bad weather threatened. It has since resumed. The drilling started in early May.

Oil began spilling from BP's leased Deepwater Horizon rig exploded on April 20 and then sank, breaking away from an oil well on the seabed 1,500 metres below. Eleven workers died.

Oil began gushing from the well, about 80 kilometres off the coast of Louisiana, at a rate of up to 50,000 barrels a day.

By the time BP managed to put a temporary cap on the damaged well in mid-July, the U.S. government estimated that 4.9 million barrels of oil had spilled.

BP has estimated that it has spent $6.1 billion US cleaning up the mess so far. It has also agreed to set up a $20-billion US fund to compensate people and businesses affected by the disaster.

With files from The Associated Press