At a briefing in New Orleans, Allen said on Wednesday he was confident a relief well preceded by a so-called "static kill" would plug the leak for good.

'Optimistic'

While the measures were not foolproof, he said engineers were "optimistic that we will get this thing done".

in depth

"This has been done before. It's not novel technology," he said.

However, exactly 100 days after the explosion on the Deepwater Horizon rig that killed 11 workers and started the leak, concerns remain over the oil's unseen effects below the ocean surface.

The high-range government estimate for the spill is that nearly 5 million barrels of oil have leaked into the ocean since the rig explosion.

Speaking to Al Jazeera, John Amos, president of SkyTruth, a company which has been monitoring the oil spill disaster, confirmed that recent satellite images had shown that the "monolithic slick" was rapidly starting to break up and dissipate.

Those findings were a positive sign, he said, but there remained a long way to go.

In particular questions remain about the large amount of oil that lingers beneath the surface of the water, as well as the long-term impact of chemical dispersants used to try and break up the slick.

"Nature has given us a big help, through evaporation, biodegradation, the mechanical breakdown of oil by storms," Amos told Al Jazeera.

"Nonetheless nature has been overwhelmed by the spill, just as our clean-up capacity has been overwhelmed.

"As we've seen in previous spills, the toxic effects of this oil can actually linger and have measurable impact for decades."