Workmen installing casing in the relief well on the floor of the Development Driller 11, on June 19 at the site of the Deepwater Horizon oil spill in the Gulf of Mexico off the coast of Louisiana. (Peter Andrew Bosch/Associated Press)

A relief well being drilled deep into the seafloor of the Gulf of Mexico to shut down the gushing well could be completed ahead of a long-set deadline of mid-August only if conditions are ideal, government and BP officials said Thursday.

National Incident Commander and retired U.S. Coast Guard Admiral Thad Allen said Thursday that the relief well is expected to intercept and penetrate the Deepwater Horizon well pipe about 5,500 metres below sea level within seven to 10 days.

But they won't know how long it will take to stop the oil until they get there. The gushing well has several rings, and oil could be coming up through multiple rings, Allen said.

Oil continues to leak from the broken wellhead at the site of the Deepwater Horizon oil well in the Gulf of Mexico, on Thursday. (BP PLC/Associated Press)The plan is to pump heavy mud and then cement into the well to overcome the upward pressure of the huge oil reservoir below.

If the oil is coming through the outer ring of the well, then they will have to pump in mud and cement to stop that layer first. Then they would have to drill through the hardened cement and repeat the process in each ring until they reach the centre pipe and do it again.

That scenario would push into the middle of August, which is the timeline the company and government officials have held to for weeks, despite repeated reports that the drilling was ahead of schedule and the oil could be stopped as soon as late July.

"If you have to exhaust all means for the ways that hydrocarbons are coming up the pipe, then that puts you into middle August," Allen said.

If the oil is only coming up the centre pipe, then it's possible to stop the leak sooner. BP spokesman Scott Dean said late July has been suggested as a completion time if everything is ideal. A single major storm is enough to cause delays. That's why the company is sticking with mid-August.

Every litre counts

The relief well is currently the best hope for staunching the oil leak sparked by the April 20 explosion aboard the Deepwater Horizon drilling rig, which killed 11 workers and began an environmental catastrophe for the region.

Shaving even days off the mid-August timeline would stop millions of litres of oil from escaping into the Gulf. The broken well has spewed between 325 million and 640 million litres of oil, according to federal estimates, or enough oil to fill about 3.4 million standard bathtubs.

The weather will have to co-operate. Lingering tropical weather that began last week with the faraway Hurricane Alex halted offshore skimming operations and caused high seas that have delayed the hookup of a third vessel expected to suck oil from the gushing well head.

Another tropical depression formed in the Gulf on Wednesday and was closely following the path of Alex to the coast at the border of Texas and Mexico. It was expected to have a minimal effect on the eastern Gulf.