ROBERT, La. -- The hypnotic video of mud, gas and oil billowing from the seafloor has become an Internet sensation as Americans watch to see whether BP's effort to plug the gusher in the Gulf of Mexico succeeds.

BP warned Friday that it could be Sunday or later before the outcome of the cliffhanger becomes clear. And scientists cautioned that few conclusions can be drawn with any certainty from watching the spillcam coverage of the "top kill." But they said the video seemed to suggest that BP was gaining ground.

In an operation that began Wednesday, BP has been pumping heavy drilling mud into the blown-out well in hopes of choking it off and putting an end to what is already the biggest oil spill in U.S. history, at anywhere from 18 million gallons to 40 million by the government's estimate.

Coast Guard Adm. Thad Allen said the denser-than-water mud was able to push down the oil and gas coming up at great force from underground, but it had not overwhelmed the gusher. The trick is to pump the mud with such force that it stops the upward flow of oil, and it is impossible to know how much mud that will take.

BP Chief Operating Officer Doug Suttles said Friday that the top kill was going basically as planned, though the pumping has stopped several times.

"The fact that it's stopped and started is not unusual," Suttles said. "We're going to stay at this as long as we need to."

He said the company also has shot in assorted junk, including metal pieces and rubber balls, that seemed to be helping to counter pressure from the well.

BP, under pressure from Congress, made available a live video feed of what is going on underwater, and about 3,000 websites were showing a version of it that the PBS "Newshour" offered for free.

On Thursday alone, show spokeswoman Anne Bell said, more than a million people watched it.

Meanwhile, President Barack Obama visited the coast Friday to see the damage as he tried to emphasize that his administration was in control of the crisis, which began April 20 when the Deepwater Horizon drilling platform blew up. Eleven workers were killed.

"I'm here to tell you that you are not alone, you will not be abandoned, you will not be left behind," he told people in Grand Isle, where the beach has been closed by gobs of oil and the frustration and anger are palpable.

"The media may get tired of the story, but we will not. We will be on your side and we will see this through."

The president noted that all may not go well in such a massive, unprecedented undertaking. Mistakes are possible, Obama said. But a lack of urgency about plugging the leak and restoring the region is not, the president declared.

"There are not going to be silver bullets or a lot of perfect answers for some of the challenges that we face," he said in front of an incongruously pristine backdrop of sparkling blue water. "But we're going to keep at this every day."

Hundreds of workers hit the beaches ahead of Obama's visit, cleaning debris from the shoreline. They left soon after the president arrived.

"This is the cleanest I've ever seen the beach," Louisiana Gov. Bobby Jindal said. "We saw a surge of activity the last couple of days. Let's hope it continues now that he's gone."