More than 24 hours into its crucial "top kill" plan, London-based BP said on Thursday it was still too early to access the success or failure of its best short-term hope of plugging a blown-out well in the Gulf of Mexico.

When it began the operation to pump thousands of barrels of heavy drilling mud down the well on Wednesday, it said results could be clear in 24 hours. On Thursday, an executive who has overseen the plug efforts for weeks said it would be another day or more.

"We're going to do the job as best we can," said Doug Suttles, BP's chief operating officer. "If it takes more than 24 hours, it takes more than 24 hours."

Pressure is mounting for the company to fix the leak. U.S. Interior Secretary Ken Salazar also has scolded the company for missing previous deadlines. BP wants to set realistic expectations on a critical procedure, said Dan Pickering, co-president of Tudor Pickering Holt & Co energy investment and merchant banking in Houston.

"Going slow and being successful is better than going fast and/or jumping to conclusions," Pickering said. After other efforts to stem or corral the leak failed, the stakes are climbing fast, he said.

"BP has indicated this has a 60-70% probability of working. If it doesn't, their credibility is weak and they're going to have a lot of very disappointed people," Pickering said.

The top kill is BP's latest effort to choke off the leak that threatens much of the Gulf Coast. The leak was discovered after a blowout preventer at the seabed failed on April 20 and the Deepwater Horizon drilling rig later exploded and sank, killing 11 workers.

Objective 'Not Yet Achieved'

The bigger of two leaks comes from the end of a broken pipe that had connected the well to the rig. A smaller leak continues from the bent pipe on top of the blowout preventer. The top kill involves pumping drilling "mud," or fluids heavier than oil, into the failed blowout preventer and then down the well. The intent is for the fluid to overcome the flow of oil and "kill" the well.

Suttles said mud was pumped into the blowout preventer for about 10 hours on Wednesday, then suspended so scientists could study pressures to gauge whether the effort was working. If pressure is high, oil and gas is still leaking. Lower pressure indicates the mud is smothering the flow, he said. He said some on BP's team saw improvement, while others were more skeptical.

While mud is pumped into the blowout preventer, some can go down into the well, while some go up and then out of a broken pipe atop the blowout preventer, where oil and gas are leaking.

"We have not stopped the flow, so the operation has not yet achieved its objective," Suttles said.

Mud pumping restarted on Thursday. Suttles said BP also would pump in "bridging agents," or more solid materials, to help block the leak. BP also may inject a "junk shot" of shredded rubber, golf balls and other materials to add more weight to push back the leak, Suttles said. Pumping may stop again to allow more evaluation of pressures, he said. "Nothing has actually gone wrong. This is a dynamic kill," he said.

The last step would be to pump in more mud and cement and cap the well to plug the leak until two relief wells being drilled finish the job, Suttles said. A live video feed of the seabed operation on BP's website had shown the larger leak, and switched on Wednesday to show the blowout preventer from the side.

On Thursday, the feed showed the smaller leak coming from the bent pipe coming out of the top of a piece of equipment atop the blowout preventer. At times an underwater robot's wrench clutched what BP spokesman David Nicholas said was a sensor. But Pickering said the live feed was there for the public and markets to see at any time, adding pressure for success.

"When you have a live webcam that anybody in the world can literally pull up and watch, you've got to be very careful about setting expectations," he said. Suttles said if the top kill failed, BP would "immediately" move to cut off the bent pipe and place a cap with a seal on the opening to corral the oil so it can be transported by pipe to a drillship at the water's surface.

BP also could place a new blowout preventer atop the failed one, he said. "It's quite a roller coaster. Every time we start a new operation, we obviously believe it could be successful," Suttles said.

See the live feed of the top kill efforts here.