WASHINGTON -- Criticism mounting, President Barack Obama vigorously defended his administration's response to the worst oil spill in the nation's history Thursday as he prepared for his second inspection tour of the battered Gulf Coast. While standing up for the effort so far, he frankly acknowledged he could have done better.

"I take responsibility. It is my job to make sure that everything is done to shut this down," Obama declared in a lengthy news conference at the White House on Thursday.

Obama's words marked a clear shift ofemphasis for an administration that previously had said it was generally "in charge" but that there were limits to what it could do -- and that oil giant BP was responsible for stopping the flow and cleaning up the disastrous damage.

"Those who think we were either slow on the response or lacked urgency, don't know the facts," Obama said at a White House news conference at which he also announced new restrictions on offshore drilling. Separately, Elizabeth Birnbaum, the head of the Minerals Management Service that oversees offshore drilling, resigned under pressure.

Obama's move to take responsibility and accept accountability was a gesture few politicians are eager to make. But with each passing day, frustration with the Obama administration has grown, and his poll numbers on the matter are dropping. The news conference and his trip to the coast today represent a more aggressive public effort by the president.

Asked about comparisons to the Bush administration's much-criticized handling of the Hurricane Katrina aftermath, the president said that was for others to judge, but he insisted his administration has been active from the start.

"This has been our highest priority" since the rig exploded, he said, making the point repeatedly.

The president announced new steps to restrict drilling, including continuing a moratorium on drilling permits for six months, suspending planned exploratory drilling off the coasts of Alaska and Virginia and ordering a halt to 33 exploratory deep-water rigs in the Gulf of Mexico.

Obama's news conference was his third this year, but his first scheduled question-and-answer session at the White House since a prime-time East Room session in July of last year. He spoke at times in personal terms.

"My job right now is just to make sure everybody in the Gulf understands: This is what I wake up to in the morning, and this is what I go to bed at night thinking about. The spill."

Obama marked out half a dozen areas where he and his administration could have done better.

They included: not moving sooner to finish reforming what he called "cozy and sometimes corrupt" relations between the oil industry and government regulators; not recognizing that those flaws continued before approving an expansion of offshore drilling; and not obtaining more quickly an accurate estimate on the amount of oil gushing from the leak.

He also said he regretted not pushing BP sooner to release underwater video footage of the leak and not realizing that oil companies did not have "their act together when it came to worst case scenarios." Though he said the government was giving the orders in the aftermath, he acknowledged that BP hasn't always done what officials have asked, for instance ignoring directions to fully explore less-toxic alternatives to the chemical dispersant being used now on the oil.

"If the question is, are we doing everything perfectly out there, then the answer is absolutely not. We can always do better," he said. "If you're living on the coasts and you see this sludge coming at you, you are going to be continually upset and from your perspective, the response is going to be continually inadequate until it actually stops. And that's entirely appropriate and understandable."

Over and over, the president sought to counter criticism that the administration was giving too much leeway to BP PLC. "Make no mistake, BP is operating at our direction," he said.

"We will demand that they pay every dime they owe for the damage they've done and the painful losses that they've cost," he said. Still, he acknowledged, "We've got to get it right."

The continuing leak, damaging coastal areas and threatening much greater harm, has been sobering for lawmakers.

Rep. John Dingell, D-Mich., noted he has supported offshore gas and oil drilling but said, "Today I am forced to come to a difficult conclusion.

"We need to establish a complete moratorium on all leasing and drilling activity until it is established that all of it was done and is being done" in compliance with environmental laws, he said.

Obama said a too-comfortable relationship between industry and government didn't change when he came into office.

Interior Secretary Ken Salazar "came in and started cleaning house. But the culture had not fully changed at MMS. And surely I take responsibility for that."

He spoke shortly after the resignation of Birnbaum, the director of the Minerals Management Service, was announced.

"I found out about her resignation today. I don't know the circumstances under which this occurred," Obama said.

A senior administration official said that Salazar informed the president Wednesday night that he had decided to replace Birnbaum after Obama told the interior secretary to make sure that every person under him was capable of doing the job.