A U.S. congressional report issued on Thursday found that the U.S. Central Command's analysis of the fight against Islamic State militants was too positive in 2014 and 2015, compared with events on the ground and other intelligence analyses.

The report came from a task force established by the Republican chairmen of the House of Representatives Armed Services Committee, Intelligence Committee and Defense Appropriations subcommittee.

It found "widespread dissatisfaction" among analysts at U.S. Central Command who felt their the superiors were distorting their products.

"What happened at CENTCOM is unacceptable - our warfighters suffer when bad analysis is presented to senior policymakers. We must continue our efforts until we fix it," Republican Representative Ken Calvert, a member of the task force, said in a statement.

The height of the Islamic State's rapid expansion was in 2014, as the militant group grabbed a swath of territory spread from Iraq into central Syria.

Patrick Evans, a Pentagon spokesman, said the Department of Defense had initiated a separate investigation into the issue, and would take no action or make any comment that could influence the inspector general's work.

However, as a general comment, he said the intelligence community routinely provides a wide range of assessments.

"Experts sometimes disagree on the interpretation of complex data, and the Intelligence Community and Department of Defense welcome healthy dialogue on these vital national security topics," Evans said.

Democrats from the House committees did not have immediate comments on the report.