The Pentagon is investigating whether military officials have improperly rewritten intelligence assessments to give a more optimistic view of the US-led campaign against the Islamic State group in Iraq, the New York Times reported Wednesday.

The inspector general probe began after at least one civilian analyst at the Defense Intelligence Agency said he had evidence that US Central Command officials were reworking intelligence report conclusions prepared for President Barack Obama and other policymakers.

Unnamed government officials detailed the claim to the Times, though it was unclear when the reports were changed and who was responsible.

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.

US officials told the Times that complaints had been made to the intelligence community's inspector general, and that the Pentagon's inspector general then took up the matter.

If the assessments were indeed skewed, it could help explain why public descriptions 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 cities like Mosul and Ramadi.

But US intelligence agencies have recently found that the group has been little weakened by the assault just as it expanded into North Africa and Central Asia, the Times said.

Last month, Obama administration special envoy retired general John Allen said "ISIS is losing," using an acronym by which the group is known.

Defense Secretary Ashton Carter agreed last week that the war was "difficult" and will take "some time," though he insisted he was "confident that we will succeed in defeating ISIL and that we have the right strategy."

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.