WASHINGTON — Wearing a blue hood to shield his identity, a former Syrian police photographer briefed a congressional committee over the summer on the photos he had smuggled out of the country to document the deaths of thousands of prisoners killed in President Bashar al-Assad’s jails. At a White House meeting, President Obama’s senior aides welcomed him as a man of uncommon courage who had revealed unspeakable atrocities.

But now the Syrian government’s most celebrated defector, who uses the pseudonym Caesar, is no longer optimistic that the United States has the will to stop the abuses that have shocked the conscience of the world.

His photographs have generated outrage but no fresh action against the Assad government. And instead of intervening militarily to support opponents of Mr. Assad, Mr. Obama is mounting airstrikes to defend Kurds, Yazidis and Turkmen in Syria and Iraq from the Islamic State.

“I completely understand how he came to the defense of two American victims killed by the extremist ISIS terror group,” Caesar said in a message earlier this month from an undisclosed location in Europe that was conveyed by the Coalition for Democratic Syria, a Syrian-American organization that sponsored his trip to Washington. “But I and millions of Syrians feel depressed when we see that the killer of thousands of prisoners is left unchecked,” he added. “I believe my cause demands action and a clear position by the president of the United States.”

 

Caesar’s complaint reflects a broader discontent within the moderate Syrian opposition that is posing a new challenge for the Obama administration’s strategy to counter the Islamic State, which is also known as ISIS or ISIL. While ruling out United States military intervention against Mr. Assad, the administration has committed to training thousands of opposition fighters in Saudi Arabia and Turkey so they can eventually defend territory in Syria that is wrested from the Islamic State’s control. But those fighters must come from the same constituency that has been increasingly troubled by the American reluctance to act more forcefully against Mr. Assad.

“There is a sense that there is discrimination against them, that the atrocities they are suffering at the hands of Assad are somehow less deserving than what is befalling other communities,” said Emile Hokayem, an expert on Middle East affairs at the International Institute for Strategic Studies. “For most of these rebels, Assad is the greatest evil, not ISIL. For the U.S., it is the opposite,” he said.

Robert S. Ford, a former American ambassador to Syria and a senior fellow at the Middle East Institute, said the administration’s twin policies of carrying out airstrikes to protect the Kurdish community in Syria while refraining from direct military support for Arab opponents of Mr. Assad might backfire.

“It will make recruiting harder for an American-trained force, and indeed, in the short term, might help ISIL gain recruits by helping it pose, however falsely, as defenders of Sunni Arabs,” Mr. Ford said.

The White House appears sensitive to the importance of Caesar’s role. Answering a letter Caesar sent in late July to the president, Mr. Obama’s deputy national security adviser, Benjamin J. Rhodes, wrote last week that the aim of the American program to train moderate opposition was to help it not only contend with the “barbaric threat of the Islamic State” but also to defend itself “from the brutality of the Assad regime.”

No one has done more to expose that brutality than Caesar. Described as mild-mannered and not particularly political, he has become a compelling element of the Syrian narrative because he emerged from the darkest side of the Assad government.

Caesar was photographing accident scenes for the military police when the Syrian conflict erupted. He and several fellow photographers soon found themselves photographing dozens of bodies a day, many of which displayed signs of torture.

Convinced that he was documenting war crimes, Caesar downloaded copies of the photos on thumb drives, sneaked them out of his office and transferred them to a hard drive, keeping a grisly record of the deaths for more than two years. But when asked to train a successor, he became alarmed that the government might be on to him, and he defected, taking a hard drive that he says documents more than 10,000 deaths.

Senior American officials say his account and the photographic record he has provided are credible.

“You see the evidence of broken bones, of the use of chemicals, of strangulation, of evisceration, of eye-gouging, of starvation,” said Stephen J. Rapp, who serves as the State Department’s ambassador at large for issues involving war crimes. “And it is all being done in a state security system.”

The photos are important for another reason. Russia’s veto power in the United Nations Security Council has prevented war crimes allegations against Mr. Assad from being referred by that body to the International Criminal Court. But if any of the victims in the photos can be identified as Syrians who also hold American or other foreign citizenship, that would make it easier for the United States or other governments to hold Syrian officials accountable in their legal systems.

According to a written agreement between the Caesar team and the State Department, the defector provided in April 26,948 photographs of people who had died in the Syrian government’s custody. In return, the F.B.I. was to analyze the authenticity of the photos and provide assessments of its findings.

The pace of that work, however, has become another source of friction. Mouaz Moustafa, the representative for Caesar in Washington, said that Caesar’s team wants to pursue legal cases against members of the Assad government in European countries and believes the F.B.I.’s efforts to identify any American and foreign citizens among the victims has been moving too slowly.

Reflecting its unhappiness, Caesar’s team has yet to give the United States the rest of the 55,000 photos and says it may seek the help of photo analysts in other governments or nongovernmental agencies. A senior American law enforcement official, who insisted on anonymity to discuss the issue, said that authenticating the photos was complex and painstaking and that there was no timetable for completing the work.

American officials have culled about 4,800 photos from the nearly 27,000 the F.B.I. received and compared them against visa and passport photos in the State Department’s database and with photos in a separate terrorism database. Representative Ed Royce, the California Republican who serves as chairman of the House Foreign Affairs Committee, and who also shares concerns about the methodical pace of the work on the photos, said that American officials had identified at least seven likely matches, though it is not clear whether any are foreign citizens.

Caesar, however, has looked to the United States for more than evidence for a possible war crimes trial. When he visited Washington last summer, he was hoping that his trip would lead to more forceful American action.

Besides testifying before Mr. Royce’s panel, Caesar sought a meeting with Susan E. Rice, Mr. Obama’s national security adviser, his aides say. Told she was not available, he scribbled a note in Arabic to Mr. Obama, which he gave to Samantha Power, the United States ambassador to the United Nations, at an emotional meeting at the State Department.

“I have risked my life and the life of my immediate family, and even exposed my relatives to extreme danger, in order to stop the systematic torture that is practiced by the regime against prisoners,” Caesar wrote. “What is it that you can possibly do to prevent the killing, especially since there are more than 150,000 prisoners in the jails of the regime awaiting this black fate?”

The White House did not want Caesar to leave Washington without a meeting, and one was organized with Mr. Rhodes and Jake Sullivan, who was serving then as the national security adviser to Vice President Joseph R. Biden Jr.

Explaining that he was speaking on behalf of the president, Mr. Rhodes praised Caesar’s courage and voiced the hope that Caesar’s photos would shame others not to help the regime.

Emad ad-Din al-Rashid, a former assistant dean at a college in Damascus who attended the meeting with Caesar, told the White House officials it would be painful if the United States joined forces with Mr. Assad to combat the Islamic State. Mr. Rhodes and Mr. Sullivan were adamant that the administration had no such intention.

But the meeting also underscored the differing expectations about the role the United States should play.

Though Mr. Rhodes mentioned the humanitarian aid the United States has provided and alluded to the effort to train moderate Syrian rebels, Mr. Rashid said he had told the White House that it risked losing the support of the Syrian public if it did not stop the Assad government from dropping barrel bombs on Syrian cities.