BP and federal government officials sought to allay the fears of residents and business owners along the Gulf of Mexico coastline that their attention on the United States' worst environmental disaster could fade now that a cement seal on the leaking wellhead is holding.

"Our cleanup activities are still substantial," said Doug Suttles, BP Plc's chief operating officer. "We still have tens of thousands of people at work out there cleaning up the shoreline."

White House energy adviser Carol Browner told NBC's Meet the Press on Sunday morning that the company will have to pay "a large financial penalty. There will also be natural resources damages claims. They will be responsible for paying to clean up the natural resources damages."

Their comments — that much still needs to be done in the oil-hit region — come on the heels of a government report just days ago that suggested oil damage in the Gulf was not as serious as previously thought.

Many including Jesse Morris, a restaurant owner in Louisiana, don't believe the report, which asserted that about 75 per cent of the leaked oil had either evaporated, dispersed or was otherwise contained.

"We still have a major, major catastrophe on our hands," Morris said.

On Thursday, BP engineers successfully plugged the hole at the bottom of the Gulf with mud and cement, a process known as a "static kill."

Later this week, they plan to pump mud and cement through a relief well to forever suffocate the source of the oil, a process known as a "bottom kill."

The spill began more than three months ago after an offshore drilling rig exploded, killing 11 workers and creating an ecological catastrophe.

BP temporarily stopped the flow of oil into the Gulf on July 15 when it placed a custom-built cap over the blown well. That was always designed to be a temporary solution until the permanent seal could be made.