WASHINGTON — President Barack Obama said yesterday he would try to block the court-ordered release of photos showing U.S. troops abusing prisoners, abruptly reversing his position out of concern the pictures would "further inflame anti-American opinion" and endanger U.S. forces in Iraq and Afghanistan.

Obama's turnabout set off immediate reactions from bloggers, both liberals who decried that he was buckling to political pressure and conservatives who agreed with the decision but said it proved the president was a flip-flopper.

The White House had said last month it would not oppose the release of dozens of photos from military investigations of alleged misconduct. But U.S. commanders in the war zones expressed deep concern about fresh damage the photos might do, especially as the U.S. tries to wind down the Iraq war and step up operations against the Taliban and al-Qaida in Afghanistan.

When photos emerged in 2004 from the infamous U.S.-run Abu Ghraib prison in Iraq, showing grinning American soldiers posing with detainees — some of the prisoners naked, some being held on leashes — the pictures caused a huge anti-American backlash around the globe, particularly in the Muslim world.

Obama, realizing how high emotions run on detainee treatment during the Bush administration and now, made it a point to personally explain his change of heart, stopping to address TV cameras late in the day as he left the White House for a flight to Arizona.

He said the photos had already served their purpose in investigations of "a small number of individuals." Those cases were all concluded by 2004, and the president said "the individuals who were involved have been identified, and appropriate actions have been taken."

The Pentagon conducted 200 investigations into alleged abuse connected with the photos that are now in question. The administration did not provide an immediate accounting of how those investigations turned out.

"This is not a situation in which the Pentagon has concealed or sought to justify inappropriate action," Obama said of the photos. "In fact, the most direct consequence of releasing them, I believe, would be to further inflame anti-American opinion and to put our troops in greater danger."

The Justice Department immediately filed a notice with the 2nd U.S. Circuit Court of Appeals of its new position on the release, including that it was considering an appeal to the Supreme Court of the 2nd Circuit’s September 2008 ruling ordering the release of the photos. The government has until June 9 to make that appeal.

The effort to keep the photos from becoming public represented for many a sharp reversal from Obama's repeated pledges for transparency in government, and in particular from his promise to be forthcoming with information that courts have ruled should be publicly available.

As such, it invited criticism from the more liberal members of the Democratic Party, who want a full accounting — and even redress — for what they see as the misdeeds of Republican George W. Bush's administration.

"The decision to not release the photographs makes a mockery of President Obama's promise of transparency and accountability," said American Civil Liberties Union attorney Amrit Singh, who had argued and won the case in question before the 2nd Circuit in New York. "It is essential that these photographs be released so that the public can examine for itself the full scale and scope of prisoner abuse that was conducted in its name."

Human Rights Watch called the decision a blow to transparency and accountability.

Republicans welcomed the change, however. A military group also said it was relieved.

"These photos represent isolated incidents where the offending servicemen and women have already been prosecuted," said Brian Wise, executive director of Military Families United.

The reactions were a reverse of what happened after Obama's decision last month to voluntarily release documents that detailed brutal interrogation techniques used by the CIA against terror suspects. Those also came out in response to an ACLU lawsuit, and his decision then brought harsh and still-continuing criticism from Republicans.

This time he's kicking the decision back into court, where his administration still may be forced into releasing the photos.

Indeed, there is some evidence that the administration has little case left.

White House spokesman Robert Gibbs said the president instructed administration lawyers to challenge the photos' release based on national security implications. He said the argument was not used before in "the most effective" way.

But the Bush administration already argued against the release on national security grounds — and lost.

"It is plainly insufficient to claim that releasing documents could reasonably be expected to endanger some unspecified member of a group so vast as to encompass all United States troops, coalition forces, and civilians in Iraq and Afghanistan," the three-judge appeals panel wrote in September 2008.

The Justice Department concluded after that that further appeal would probably be fruitless, and last month, Gibbs said the administration felt "compelled" to act on that conclusion. Thus, the administration assured a federal judge that it would turn over the material by May 28, including one batch of 21 photos and another of 23 images. The government also told the judge it was "processing for release a substantial number of other images," for a total expected to be in the hundreds.

White House Chief of Staff Rahm Emanuel said yesterday that Obama always felt uncomfortable with that outcome and pressed his team to find other recourse. After first believing all avenues were shut, they concluded there were other options — both in the amount of time left and the legal arguments — and that led to the decision the White House announced, Emanuel said.

Obama said yesterday, "I want to emphasize that these photos that were requested in this case are not particularly sensational, especially when compared to the painful images that we remember from Abu Ghraib."

Still, he said he had made it newly clear: "Any abuse of detainees is unacceptable. It is against our values. It endangers our security. It will not be tolerated."