Two days after BP began a risky effort to stop a gushing oil well in the Gulf of Mexico, company officials said the operation was going as planned but offered few details, leaving it unclear whether America's worst oil spill would end any time soon.

BP warned it could be today or later before it becomes clear whether its bid to plug the well through an effort known as a "top kill" is working.

Experts said they could see incremental progress at best from BP's "spillcam" of mud, gas and oil billowing from the seafloor. The hypnotic video has become an internet sensation.

Scientists say the images may offer clues to whether BP is getting the upper hand in its struggle to contain the oil, said Tony Wood, director of the National Spill Control School at Texas A&M University in Corpus Christi. If the stuff coming out of the pipe is jet black, it is mostly oil and BP is losing. If it is whitish, it is mostly gas and BP is also losing.

If it is muddy brown, as it was much of Friday, that may be a sign that BP is starting to win, he said. That "may in fact mean that there's mud coming up and mud coming down as well," which is better than oil coming out, Wood said.

Philip W Johnson, an engineering professor at the University of Alabama, said the camera appeared to show mostly drilling mud leaking from the well on Friday morning, and two of the leaks appeared a little smaller than in the past, suggesting the top kill "may have had a slight but not dramatic effect".

But Bob Bea, a professor of engineering at University of California at Berkeley who has studied offshore drilling for 55 years, said that what he saw didn't look promising.

He likened the effort to pushing food into a reluctant baby's mouth — it only works if the force of the stuff going down is more than the force of what's coming up.

"It's obvious that the baby's spitting the baby food back" because the pressure pushing up from the well is stronger, Bea said.

The top-kill operation began on Wednesday, with BP pumping heavy drilling mud into the blown-out well in an effort to choke off the source of the spill, which has released anywhere from 18 million gallons to 40 million gallons of oil by the government's estimate.

On Friday, President Barack Obama arrived at the Gulf of Mexico to sample for himself the destruction of coastal eco-systems and the despair of those whose livelihoods depend on them.

In Grand Isle, an island in Louisiana, Obama ran into the anger of a population which can't quite believe that the oil spill has happened and that, 40 days later, it still isn't over.