BP's chief executive Tony Hayward admitted Monday that he had underestimated the possible environmental impact of the massive oil spill in the Gulf of Mexico.

Hayward, who visited oil-soaked Fourchon Beach Monday, told journalists that he was "devastated" by what he was seeing at Fourchon Beach, one of the few sandy beaches in Louisiana whose vast marchland is now threatened by oil spill.

Hayward said in an earlier interview with a British media that the environmental impact of the spill would likely be "very, very modest."

"As I said, it's clear that the defense of the shoreline, at this point, has not been successful," Hayward admitted. He said the spill now is "clearly a major reputational issue for BP."

"We are going to do everything in our power to prevent any more oil from coming ashore and we will clean every last drop up and we will remediate all of the environmental damage," he said.

At least 6 million gallons of crude oil have spewed into the Gulf of Mexico since the explosion and sinking of the Deepwater Horizon rig, which was leased by BP, in late April, according to a Coast Guard and BP estimate.

But some scientists believe the spill has already surpassed the 10.8 million gallons of oil spilled off Alaska during the 1989 Exxon Valdez oil spill, the largest oil spill to date in U.S. waters.