BP Chief Executive Officer Tony Hayward said Wednesday the newly launched "top kill" operation to plug the oil leak in the Gulf of Mexico was going according to plan.

However, he said there will be 24 hours before authorities can decide whether the newest oil-containment method is working.

He also cautioned against trying to reach any conclusions based on the video shots from the ruptured wellhead a mile below the sea surface.

"It's unlikely to give us any indication of what's really going on," he said from a command center in Houston, nearly four hours after the launch of the "top kill" effort.

"Increases or decreases are not an indicator of success or failure at this time. We will be continuing for at least another 24 hours, and it will be 24 hours before we will know whether or not this has been successful," he said.

BP started the "top kill" procedure Wednesday afternoon.

At 1 p.m. CT, it began pumping 50,000 pounds of thick, viscous fluid twice the density of water into the site of the leak, BP said.

If all goes according to plan, it will be followed by either heavy mud or cement that increases the hydrostatic head on the well, stopping the flow of the well.

BP is pinning its hope on this new method after several failed attempts to stop the month-long oil spill in the Gulf of Mexico.

The "top kill" has never been tried 1,500 meters beneath the sea. Hayward said the method has a 60 percent to 70 percent chance of success.