Oil floats on the surface in Pass A Loutre near Venice, Louisiana May 26, 2010. REUTERS/Sean Gardner

HOUSTON (Reuters) - BP Chief Operating Officer Doug Suttles said on Wednesday it appears drilling mud, not oil, was gushing from a ruptured undersea well six hours into an effort to halt a growing oil spill.

“What you’ve been observing coming out of the top of that riser is most likely mud,” Suttles said at a news conference broadcast from a Louisiana command center. “We can’t fully confirm that because we can’t sample it. And the way we know we’ve been successful is it stops flowing.”

Residents along the Gulf Coast have been watching video of the leaking well since the “top kill” began about 1 p.m. CDT (2 p.m. EDT). The effort involves pumping tons of drilling mud down the well bore to overcome the oil and gas flowing out.

Suttles and U.S. Coast Guard Admiral Mary Landry said they were cautiously optimistic about the effort to stop the leak that began with a drilling rig explosion April 20. The leak threatens the ecology of the U.S. Gulf of Mexico and the economies of four U.S. states.

“We’ve all been here a long time and been trying a lot of things and ridden a roller coaster,” Suttles told reporters in Robert, Louisiana, and listening over the phone and Internet. “I think we just need to take the next 24 hours and see what the results are.”

The goal is to halt the flow of oil long enough so cement can be pumped in to shut it off.

The mud, a substance specially created for the oil drilling industry and used in all wells drilled, weighs twice as much as water and is being pumped in by 30,000 horsepower compressors.

“Ultimately, what we need to see is that the well can’t flow to surface,” Suttles said. “That will be the way we know it’s successful.”

The mile-deep underwater gusher began flowing after the Deepwater Horizon drilling rig exploded on April 20, claiming the live of 11 workers.