By Kristin Roberts

WASHINGTON, May 17 (Reuters) - The U.S. military needs more time in Iraq than the American public is willing to invest and that difference is "troubling," the top U.S. Marine Corps official said on Thursday.

"The difference in the time we in uniform need for success in Iraq and the amount of time our countrymen are prepared to invest is a disconnect that's troubling," Marine Corps Commandant Gen. James Conway said.

Conway said military operations against insurgencies can last as long as nine to 10 years.

"I think there is less of an appetite in our country than we, the military, might think we need to sustain that kind of effort over that period of time," he said.

Conway would not comment on how long U.S. troops might stay in Iraq, or how long he thinks they should stay to secure the country and help Iraq build its military.

But he said the United States is operating on a time frame far shorter than that of al Qaeda.

"You know, the bad guys' timeline's 100 years. Ours is probably somewhere short of two at this point," he said.

The U.S. force now totals about 147,000 and will reach 160,000 in June as the Pentagon increases troop levels for a security crackdown focused on Baghdad.

That operation is seen as a last-ditch effort to halt Iraq's descent into full-scale civil war, but it has had little impact on the level of violence in the capital city.

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