WASHINGTON — Thrown on the defensive, President Barack Obama acknowledged that his administration could have done better in dealing with the biggest oil spill in the nation’s history and misjudged the industry’s ability to cope with a worst-case scenario. Obama will make his second tour of the battered Gulf Coast on Friday.

MORE: BP RESTARTS ‘TOP KILL’ PROCESS



“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. As he spoke, well owner BP struggled anew to plug the blown well that exploded five weeks ago, killing 11 workers and sending millions of gallons of polluting oil gushing out.

WATCH VIDEO OF THE SPILL

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, 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.

Taking control carried its own political risks for Obama, because any failure to stop the gusher would then belong to the president. But Obama could suffer politically if his administration was 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 had been slow to act and had 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.”

A team of U.S. scientists on Thursday significantly raised the estimate of how much oil has been leaking from a damaged well into the Gulf of Mexico. The figures signal that the disaster is at least as big as the Exxon Valdez spill two decades ago, and could perhaps double it in size.

Between 12,000 and 19,000 barrels a day are estimated to be spilling into the waters of the Gulf, said U.S. Geological Survey director Marcia McNutt, the leader of an inter-agency team created to measure the size and rate of the spill following criticism that a previous estimate of 5,000 barrels a day was inaccurate.

The announcement comes as Coast Guard Admiral Thad Allen said Thursday that BP PLC’s effort to stop the flow of oil from a broken well in the Gulf of Mexico has so far “stabilized the wellhead” and stopped the oil and gas from coming up.

But Adm. Allen and a BP executive, in separate appearances Thursday, cautioned that the so-called top kill operation to seal the well, and stop a gusher of crude fouling the Louisiana coast, isn’t complete.

The U.S. scientist group, which comprised scientists from the U.S.

Geological Survey, the National Oceanic and Atmospheric Administration, the University of Washington, the University of Texas and other institutions, calculated the amount of oil on the surface to range between 130,000 and 270,000 barrels of oil as of May 17.

A similar volume of oil had already been burned, skimmed or evaporated, McNutt said in a conference call, putting the total size of the spill at 260,000 to 540,000 barrels as of that date.

In 1989, the Exxon Valdez released 11 million gallons, or about 260,000 barrels of oil, in Alaska’s Prince William sound, creating one of the worst environmental disasters in U.S. history and triggering an onslaught of oil-spill legislation.

The flow rate estimate put forward by the scientist group is the overlap of the results seen by two study groups, one of which studied oil on the surface and calculated that the spill rate ranged between 12,000 barrels and 19,000 barrels a day. The other group, which analyzed the underwater plume, gauged the leak rate at 12,000 to 25,000 barrels a day.

The Associated Press and The Wall Street Journal contributed to this story.