BP has hit a snag in its third attempt to stop the flow of oil from a rig that exploded and sank in the Gulf of Mexico more than three weeks ago, U.S. Interior Secretary Ken Salazar said Saturday.

Technicians have spent more than 24 hours trying to thread a 15-centimetre tube into the 53-centimetre pipe that is designed to siphon the oil to a tanker at the water's surface. A stopper surrounding the tube would prevent oil from leaking into the ocean.

But crews have so far been unable to complete work that began early Friday. The technicians are using joysticks to guide deep-sea robots to thread the tube through the pipe.

"There was a problem. They had to reconfigure. They are back down again ... trying to get it inserted," Salazar told reporters during a briefing at a bird rescue facility in Louisiana.

Neither Salazar nor BP spokesperson Jon Pack offered further details about the delay.

"We've never done such operations before and we need to take our time to get it right," Pack told the Associated Press in an email Saturday after learning of Salazar's comments.

The threading work follows an attempt last week to cut off the leak with a 100-ton containment box that was lowered to the ocean floor. But that solution had to be abandoned after water crystals covered the box.

Company officials hope that the tube will finally contain the flow of oil, while chemical dispersants will help keep the oil from reaching the water's surface and spreading. Crews received word Friday that federal officials gave the okay to use the chemical dispersants, which have never before been used underwater.

"We didn't cross this threshold lightly," Coast Guard Rear Adm. Mary Landry said of the dispersants. "This is a tool that will be analyzed and monitored."

BP chief executive Tony Hayward told BBC Radio Saturday that the best-case scenario is crews get the leak under control within a week to 10 days. The worst-case scenario is "more than that, and it's impossible to say how much more," Hayward said.

The sunken rig has been pouring at least 795,000 litres of oil into the Gulf of Mexico every day since it exploded on April 20, killing 11 workers.

Environmental concerns

The effort to stop the pollution with a flood of chemicals is not sitting well with some eco-activists. BP's choice of dispersant is Corexit. Of the 18 approved products on the market, government tests show Corexit to be among the most toxic and the least effective.

But officials say they are using the product because there was a large supply available immediately.

"We have to be able to supply these in sufficient quantities," said BP CEO Doug Suttles.

BP says it has ordered another 800,000 gallons of the product.

Some are questioning BP's motives behind the decision to use Corexit. Corexit is made by a company called Nalco. One of the directors of Nalco is Rodney Chase -- a man who spent 38 years at BP. Nonetheless, chemicals used to break up the oil seem to have been more effective than efforts to contain it.

After the blast, U.S. President Barack Obama announced a 30-day review of safety procedures for oil wells and rigs before any new leases can be granted.

Hayward said Saturday that U.S. rules for offshore drilling are already "very rigorous," but he said regulations will likely change after the explosion.

"You can't have an incident of this seriousness and not expect significant changes as a consequence," Hayward said.

With files from The Associated Press and a report from CTV News' Tom Walters.