By injecting solid objects as well as heavy drilling fluid into the stricken well leaking oil into the Gulf of Mexico overnight, engineers appeared to have stemmed the flow of oil, Adm. Thad W. Allen of the Coast Guard, the leader of the government effort, said on Friday morning. But he stressed that the next 12 to 18 hours will be 'very critical' in permanently stanching what is already the worst oil spill in U.S. history. Allen, who spoke on ABC's 'Good Morning America,' said the biggest challenge will be to sustain the 'top kill' effort, which involves pumping material into the well to counteract the upward pressure of the gushing oil so that the well can be sealed. 'They've been able to push the hydrocarbons and the oil down with the mud,' he said, referring to the heavy drilling fluid. 'The real challenge is to put enough mud into the well to keep the pressure where they can put a cement plug over the top.' The top kill effort has proceeded in fits and starts. BP officials, who along with government officials created the impression early on Thursday that the strategy was working, disclosed later that they had stopped pumping the night before when engineers saw that too much of the drilling fluid was escaping along with the oil. It was the latest setback in the effort to shut off the leaking oil, which federal officials said was pouring into the gulf at a far higher rate than original estimates suggested. If the new estimates are accurate, the spill would be far bigger than the Exxon Valdez disaster in 1989. With President Barack Obama planning to visit the gulf on Friday, Tony Hayward, BP's chief executive, said on 'Good Morning America' that efforts to plug the well were 'going pretty well according to plan.' 'Much of the volume you see coming out of the well in the last 36 hours is mud,' he said, referring to live video shots of the oil leak. He said that overnight, workers pumped what is known as 'junk shot,' a mix of more substantial materials into the well, like golf balls and shredded tires, and he said they would follow with more mud later today. The junk shot serves as a 'bridge,' he said, for the injections of mud to strengthen its ability to counteract the leaking oil. While he was optimistic, Hayward gave the effort a 60 to 70 percent chance of success because it has never been attempted in water this deep. 