A new Pentagon report says the U.S. and its allies have made "tangible progress" in the Afghanistan war, providing the needed momentum to transfer control of seven regions to Afghan forces this summer.

The report gives the first assessment since the infusion of 30,000 more American troops to the battlefield late last fall.

The semi-annual report to Congress sounds a more optimistic tone than previous reviews. But it also describes difficult challenges ahead, including the lack of military trainers and the slow political development that could threaten the progress.

The review comes as Gen. David Petraeus, the top U.S. commander in Afghanistan, is about to deliver options to Defense Secretary Robert Gates for the size and pace of the withdrawal of American troops that is expected to begin in July.