More than 50 US intelligence analysts have complained that senior military officials altered reports so as to downplay the strength of Islamic State and Al-Qaeda's branch in Syria, the Daily Beast reported Thursday.

The online news outlet said senior officials at US Central Command changed the reports to bring them into line with the White House's rosier public view that the United States was winning the war against the militant groups.

"The cancer was within the senior level of the intelligence command," the report quoted a defense official as saying.

It said the revolt came to a head when two senior analysts at Centcom signed and sent a written complaint to the Defense Department inspector general in July alleging that reports, including some presented in briefings to President Barack Obama, portrayed the groups as weaker than the analysts believed they were.

Another 50 analysts supported the complaint, the Daily Beast said, citing 11 unnamed individuals knowledgeable about the details of the report.

Peter Cook, the spokesman for Secretary of Defense Ashton Carter, declined to comment on details in the report, but he did say Carter wanted only undiluted intelligence reports.

"He has made clear that he expects candid intelligence analysis to come his direction, for folks to call it like they see it," Cook told reporters.

"Unvarnished, transparent intelligence is what this secretary expects on a daily basis."

Under a directive by the Office of the Director of National Intelligence, which oversees the 17 US intelligence agencies, analytical assessments "must not be distorted" by a particular audience, agenda or policy view.

If the assessments were indeed skewed, it could help explain why public perceptions of progress against the militants has varied.

Since the US-led bombing campaign of the Islamic State began in Iraq a year ago, and subsequently in Syria, Iraqi security forces have retaken some territory previously seized by the group but not major Iraqi cities like Mosul and Ramadi.

But US intelligence agencies have recently found that the group has been little weakened by the air strikes and was expanding into North Africa and Central Asia.

Syria's conflict began with anti-government demonstrations in March 2011.

But after a bloody crackdown by the ruling regime, it spiraled into a multi-front civil war that has left more than 240,000 people dead.