The top U.S. commander in Afghanistan on Monday called for a new strategy to defeat the Taliban, calling the security picture "serious" but not irreversible. 

Gen. Stanley McChrystal sent his strategic review of the Afghan war to the Pentagon on Monday. 

He did not ask for more troops but is expected to do so in a separate request, two NATO officials said on condition of anonymity because they were not authorized to speak on the matter. 

Increasing U.S. forces is a hot-button issue that could ignite furious debate in Washington on the U.S. military's future in an increasingly unpopular war. Some Democratic senators have increased calls for a timeline to draw down troops. 

NATO spokesman James Appathurai told FOX News Radio that McChrystal's report is an "assessment" and not a "final change of strategy," and still has to be reviewed by U.S. allies. 

"We need to show the American people, but also the people of the 41 other countries contributing to this operation, progress," he said. 

U.S. Secretary of Defense Robert Gates ordered the 60-day review to size up the rapidly deteriorating security situation in Afghanistan as Taliban attacks rise and U.S. deaths spiral upward. 

"The situation in Afghanistan is serious, but success is achievable and demands a revised implementation strategy, commitment and resolve, and increased unity of effort," McChrystal said in a statement Monday. 

A NATO statement said McChrystal's assessment seeks to implement President Obama's strategy "to reduce the capability and will" of insurgents and extremists, including Al Qaeda, and support the growth and development of Afghan security forces and Afghan governance. 

McChrystal, who took over command in Afghanistan on June 15, delayed the release of the review so that it would not interfere with Afghanistan's Aug. 20 presidential election. 

Obama ordered 21,000 additional U.S. forces to Afghanistan this year in part to help protect voters who wanted to cast ballots. Election officials are counting votes and sorting through hundreds of fraud complaints. With ballots from a third of polling stations counted, President Hamid Karzai has 46 percent of the vote and a strong lead over challenger Abdullah Abdullah

FOX News' Radio's Mike Majchrowitz and The Associated Press contributed to this report.