Gates, speaking at the National Defense University in Washington, said that while it is unlikely the United States would launch another war to force a regime change "anytime soon," he said U.S. forces will likely need to stay in both countries to avoid "a disastrous blow to our credibility, both among our friends and allies and among potential adversaries," CNN reported.

Gates predicted that no matter who becomes the next U.S. president in November, troops will likely stay in Iraq and increase in Afghanistan, which he called the more difficult of the two countries to operate in, the broadcaster reported.

"Given its terrain, poverty, neighborhood and tragic history, Afghanistan in many ways poses an even more complex and difficult long-term challenge than Iraq -- one that, despite a large international effort, will require a significant American military and economic commitment for some time," Gates said.

CNN said Gates also warned of "failing states" lacking a cohesive government, saying they, rather than aggressive advanced nations, pose the bigger threat as far as possible terrorist attacks against the United States.