More than half the oil released from a busted BP well remains in the Gulf of Mexico, a presidential panel was told Monday, as the US pointman lamented a "dysfunctional" response to the disaster.

Interior Secretary Ken Salazar meanwhile told the bipartisan commission that the spill had bolstered a drive to reform federal regulations for offshore drilling, promising that lessons were learnt.

In an ominous sign for Gulf residents, however, oceanographer Ian MacDonald told the probe that while much of the oil was dispersed, evaporated or removed by burning and skimming, the "remaining fraction -- over 50 percent of the total discharge -- is a highly durable material that resists further dissipation."

His assessment implied some 2.5 million barrels of oil -- or 105 million gallons -- was still embedded in the fragile ecosystem, out of the estimated 4.9 million barrels that gushed into the Gulf during the 87 days before the well was capped.

"Much of it is now buried in marine and coastal sediments," MacDonald warned, adding there was "scant evidence for bacterial degradation of this material prior to burial."

The analysis was in contrast to statistics from US officials in early August that said about 75 percent of the oil spilled from the well had disappeared.

Retired coast guard admiral Thad Allen, US pointman for the spill response, earlier acknowledged that confused perceptions -- for both the US public and local public officials -- over exactly who was in charge, meant that "procedures that worked terrific for the last 20 years became dysfunctional."

Critical action to prevent oil from washing onto the fragile Gulf Coast, stop the flow of crude and clean it off the shoreline -- in the wake of the April 20 explosion that crippled BP's Deepwater Horizon drilling rig off Louisiana -- were all impacted by the confusion, Allen said.

But Doug Suttles, BP's chief operating officer for exploration and production and one of the most public faces of the energy giant throughout the crisis, insisted there was no mistaking who made the calls.

"It was always clear to me that the Coast Guard was in charge," he told the panel.

Billy Nungesser, the outspoken president of Plaquemines Parish in southern Louisiana, told the panel the problem persists months after the oil stopped flowing: "This late in the day, I still can't tell you who's in charge."

Known for his passionate tirades against the government and BP as the spill impacted the local economy and coastline, Nungesser also told the panel: "I'm still angry."

Hundreds of miles of coastline from Texas to Florida were sullied by the devastating spill, killing wildlife and choking local communities that rely on industries such as tourism and fishing.

Michael Bromwich, head of the revamped Bureau of Ocean Energy Management, Regulation and Enforcement, joined Salazar to discuss the offshore drilling moratorium that has been criticized by oil companies that work in the Gulf.

He warned that even after the freeze is lifted, "you won't see drilling the next day, or even the next week. It's going to take time."

"The lessons that have been learnt in the last six months we are going to be applying in the new regulatory framework," Salazar said.

US Coast Guard Rear Admiral Mary Landry earlier told the probe that the government "absolutely felt there could be a reduction" in the use of subsea chemical dispersants, but said in the early weeks of the spill there was a need to mitigate the coastal impact and amount of oil at the Deepwater Horizon site.

"Over time we met that goal of reducing dispersants," she added.

By the time the well was capped after more 12 weeks, however, some 1.8 million gallons of of the chemicals had been used.

Dispersants were deemed "generally less toxic than oil... and has been known to biodegrade over days and weeks, much quicker than the oil," Environmental Protection Agency Administrator Lisa Jackson said.

The long-term effects of the use of dispersants and the massive quantities still "warrant caution," she also warned the commission.

Earlier this month US officials declared the broken well to be finally capped, but BP still faces a long uphill battle to clean up the Gulf, along with a litany of lawsuits, billions of dollars in fines, and shareholders angered by the firm's instability after its share price more than halved.

BP has pledged to continue "remedying the harm that the spill caused to the Gulf of Mexico, the Gulf Coast environment and to the livelihoods of the people across the region."