WASHINGTON  President Obama uttered three words on Thursday that many of his 43 predecessors twisted themselves into knots trying with varying degrees of success to avoid: “I was wrong.”

He strode into the East Room to mount a robust defense of his handling of the largest oil spill in American history, reassuring the nation that he was in charge and would do “whatever is necessary” to stop and clean up the BP leak in the Gulf of Mexico. But by the time he walked out an hour later, he had balanced that with a fairly unusual presidential self-critique.

He was wrong, he said, to assume that oil companies were prepared for the worst as he tried to expand offshore drilling. His team did not move with “sufficient urgency” to reform regulation of the industry. In dealing with BP, his administration “should have pushed them sooner” to provide images of the leak, and “it took too long for us” to measure the size of the spill.

“In case you’re wondering who’s responsible, I take responsibility,” Mr. Obama said as he concluded the news conference. “It is my job to make sure that everything is done to shut this down. That doesn’t mean it’s going to be easy. It doesn’t mean it’s going to happen right away or the way I’d like it to happen. It doesn’t mean that we’re not going to make mistakes. But there shouldn’t be any confusion here. The federal government is fully engaged, and I’m fully engaged.”