by Jennifer Loven and Tom Raum

WASHINGTON (AP) – On the defensive more than five weeks into the nation's worst-ever oil spill, President Barack Obama insisted Thursday that his administration, not oil giant BP, was calling the shots in the still-unsuccessful response.

"I take responsibility. It is my job to make sure that everything is done to shut this down," Obama declared at a news conference in the East Room of the White House. The Gulf of Mexico oil spill dominated the hour-long session.

He called the spill an "unprecedented disaster" and blasted a "scandalously close relationship" he said has persisted between Big Oil and government regulators.

Obama announced new steps to deal with the aftermath of the spill, including continuing a moratorium on drilling permits for six months. He also said he was suspending planned exploration drilling off the coasts of Alaska and Virginia and on 33 wells under way in the Gulf of Mexico.

The president's direct language on being in charge of the spill response, which he repeated several times, marked a change in emphasis from earlier administration assertions that, while the government was overseeing the operation, BP had the expertise and equipment to make the decisions on how to stop the flow.

As recently as Monday, the top federal official in charge of dealing with the oil catastrophe, Coast Guard Adm. Thad Allen, declined to broadly say the federal government was "in charge." Instead, when asked about that, Allen told reporters that BP was responsible for the cleanup and the government was accountable to make sure the company did it. "I would say it's less a case of 'in charge,'" Allen said when asked about that phrase.

Yet with each passing day, public frustration with Obama's administration has grown, and his poll numbers on the matter are dropping.

Claiming control carries its own political risks for Obama, because any failure to stop the gusher will then belong to the president. But he could suffer politically if his administration is seen as falling short of staying on top of the problem or not working hard to find a solution.

"The American people should know that from the moment this disaster began, the federal government has been in charge of the response effort," Obama said. He was reacting to criticism that his administration has been slow to act and has left BP in charge of plugging the leak.

Obama said many critics failed to realize "this has been our highest priority."

"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."

"There shouldn't be any confusion here. The federal government is fully engaged," he said, underscoring his central point.

As he spoke, BP worked furiously to pump mud-like drilling fluid into the blown-out well.

It was an untested procedure but seemed to be working, officials said Thursday, even as new estimates showed the spill has surpassed the Exxon Valdez in Alaska as the worst in U.S. history.

Obama said while the "top kill" procedure being used by BP demonstrated his administration's willingness to try "any reasonable strategy" to stop the gusher, the process "offers no guarantee of success."

Asked about inevitable comparisons between his handling of the disaster with his predecessor's handling of Hurricane Katrina in 2005, Obama said:

"I'll leave it to you guys to make those comparisons and make — and make — and make judgments on it, because — because what I'm spending my time thinking about is how do we solve the problem?

"And I'm confident that people are going to look back and say that this administration was on top of what was an unprecedented crisis," he added.

"This has been our highest priority," he said. He conceded that "people are going to be frustrated until it stops."

As an example of the government's hands-on approach, Obama said that BP had wanted to drill a single "relief" well in an effort to eventually stop the leak in several months if all else failed. Instead, the administration insisted on two relief wells being drilled, Obama said.

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."

He denounced what he called "the oil industry's cozy and sometimes corrupt" ties with government regulators.

Sen. Frank Lautenberg , D-N.J., a critic of offshore drilling, said Obama had taken an important step to halt the most imminent environmental threat to the Atlantic coast, but he said the danger will remain until there is a permanent ban on drilling in the Atlantic.

"BP's oil catastrophe in the Gulf is a wake-up call for our nation. Giving Big Oil more access to our nation's waters will only lead to more pollution, more lost jobs and more damage to our economy," Lautenberg said.

Obama said the federal government "has acted consistently with a sense of urgency" on the spill. But, he acknowledged a "sense of complacency on the government's part in planning how to deal with the worst-case scenario" before it happened.

He said a cozy 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."

But, he added, "there is no evidence some of the corrupt practices that took place earlier took place under the present administration's watch."

He spoke shortly after the head of the troubled agency that oversees offshore drilling, Minerals Management Services Director Elizabeth Birnbaum, resigned under pressure.

"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 had informed the president Wednesday night that he had decided to replace Birnbaum at MMS, after Obama told the interior secretary to make sure that every person under him was capable of doing the job he had. However, Obama was not aware of how the replacement was carried out Thursday morning, said the official, speaking on condition of anonymity to describe private conversations.