The head of U.S. Central Command in Afghanistan warned that months of fighting lie ahead in what will likely be the biggest military operation there since the American-led invasion of 2001.

Gen. David Petraeus said the effort will center on 10 percent of districts where about 70 percent of Afghanistan''s violence occurs.

Petraeus was in San Francisco as part of a national speaking tour — he gave a talk in Seattle on Wednesday night. He was greeted warmly at the Marines'' Memorial Club by a mostly male, pro-military crowd that included many veterans.

During his hourlong speech, which included a PowerPoint presentation, Petraeus warned of a tough fight as Marines have recently gone deeper into Taliban areas of southern Afghanistan to "reverse the cycle of violence" there.

He described it as, "the longest campaign."

Petraeus added that it will take a "sustained and substantial commitment" to prevent the Taliban from making the area a hot spot for terrorists. He said it''s especially important for troops to have a presence prior to presidential elections there next month.