Leaders at the US military's Central Command did not falsify intelligence relating to the fight against the Islamic State group in Iraq and Syria, a Pentagon report said Wednesday.

Intelligence analysts in 2015 had complained that senior military officials altered assessments so as to downplay the strength of IS, and an interim congressional report released last year found frequent attempts to distort or suppress intel.

But a mammoth new report, compiled by the Pentagon's Office of the Inspector General, found it could not substantiate the most serious allegation -- that intelligence was falsified.

"Only a few witnesses described intelligence assessments as false, and they did not provide specific examples that supported the allegation," the report states.

Intelligence assessments have become a highly politicized issue in the United States, and Republican critics of the anti-IS campaign, formulated by the administration of Barack Obama, were keen to pounce on anything indicating problems with the operation.

Obama's successor, President Donald Trump, has also weighed in on intelligence matters, pooh-poohing the assessment that Russia was behind the hack of the Democratic National Committee.

The US Central Command, better known as CENTCOM, is responsible for running America's military involvement across the Middle East and in Afghanistan.

However, the report also found management problems, and a "widespread perception" that intelligence leaders "were distorting intelligence to present a more positive view of the success of the (Iraqi Security Forces) and a more negative view of the success" of IS.

The Inspector General was also "struck by the inadequate efforts" by CENTCOM to tackle the problem.