Commander of U.S. Central Command, Gen. Lloyd Austin II. (Allison Shelley/Getty Images)

The Pentagon’s inspector general is investigating an internal complaint that U.S. military intelligence analysts have painted an overly optimistic assessment of the war against the Islamic State, officials said Wednesday.

The complaint was filed in recent weeks by a Defense Intelligence Agency civilian analyst who raised concerns that assessments developed by the military command overseeing the war were overstating the progress of a conflict seen by U.S. spy agencies as a stalemate, officials said.

The dispute centers on intelligence assessments of the war generated by the U.S. Central Command, which oversees military operations in the Middle East. Although Central Command leaders have cautioned in public that the fight will take years, they have described the U.S.-led bombing campaign as being on the right track, saying that the Islamic State has lost thousands of fighters and control over some territory in Iraq and Syria.

A spokeswoman for the Pentagon inspector general and a spokesman for the Defense Intelligence Agency declined to comment. The existence of the investigation by the Defense Department’s inspector general was first reported Tuesday night by the New York Times.

Disagreements among U.S. intelligence agencies are common, and it was unclear exactly what had been alleged by the DIA analyst, who was assigned to work for the Central Command.

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But a recent DIA assessment concluded that the Islamic State had not been substantially degraded by a year-long campaign of U.S. airstrikes and ground operations by the Iraqi military. Those findings, which surfaced publicly after they were shared with members of Congress, angered Central Command officials and others in the military who have chafed at pessimistic views of the war’s progress.

Air Force Col. Patrick Ryder, a Central Command spokesman, declined to comment on the investigation or the complaint.

In an e-mailed statement, he noted that intelligence analysts routinely produce “a wide range of subjective assessments” and that “it is ultimately up to the primary agency or organization whether or not they incorporate any recommended changes­ or additions.”

A former senior intelligence official said that there are low barriers to filing a bias complaint or launching an inspector general probe, and that neither step would necessarily suggest that Pentagon leaders see evidence of wrongdoing. “The threshold is so low,” the former official said.

Allegations of skewed intelligence assessments have come under enhanced scrutiny since the George W. Bush administration relied on flawed intelligence to justify the U.S.-led invasion of Iraq in 2003.

The former senior U.S. intelligence official said there is constant tension between analysts assigned to military commands and those in Washington. “Those who are side by side with the operators . . . see the glass half full,” the former official said. “That tension you will never get down to zero.”

The Central Command’s top intelligence officials — Maj. Gen. Steven Grove and his civilian deputy, Greg Ryckman — are highly regarded in the intelligence community, the former official said. “I find it hard to believe that the two of them would say scratch that out, put this in” to alter assessments of the war against the Islamic State, he said.