The top U.S. military officer said Thursday he could not estimate with confidence how long American forces will have to stay in Iraq.

"I really do believe it's unknowable," said Gen. Richard Myers (search), chairman of the Joint Chiefs of Staff, in an interview with a group of reporters. "If I gave a good professional estimate, then that would be a standard that people would point to and, knowing that we can't know it perfectly, we'd get hammered."

Myers also said he was not worried about the possibility that the Iraqis will ask U.S. forces to leave the country once Iraqi sovereignty is restored this summer: "That is not a particular concern."

Pressed on the matter of how long U.S. forces might stay, Myers said, "There is not a range in my own mind."

"We're going to have to let events dictate" when the military can leave, he said.

His statement echoed a White House report presented to Congress this week. "It is not possible to know at this time either the duration of military operations or the scope and duration of the deployment of U.S. armed forces necessary for the full accomplishment of our goals," the report said.

For planning purposes, the Army (search) is assuming it will have to keep roughly 100,000 troops in Iraq for at least another two years, other officials have said.

Myers said that if he and the top U.S. commanders in Iraq were to "sit around a table, I think we could draw out a pretty good diagram of where we think we're going to go" with winding up the Iraq operation. "So we think about that, and we have notions or thoughts on that."

On the other hand, he said, publicly revealing those internal deliberations would be a mistake.

"Actually, the things we've sat around and talked about before have been wrong on every count," he said with a chuckle. "So that's probably another reason why I don't want to" discuss it. Although he did not mention it, U.S. officials had assumed when Baghdad fell to U.S. forces last April that tens of thousands of troops could be withdrawn within a few months.

He also said the rotation of tens of thousands of U.S. forces into and out of Iraq, which began in January and will be largely completed in April, has been carried out without any attacks on troops entering and leaving the country.

Of the approximately 110,000 troops designated to replace the roughly 130,000 who have been there since the war began or shortly afterward, about 40,000 troops have arrived for duty, Myers said. About 30,000 troops have returned home, he said.

Asked about the hunt for Al Qaeda leader Usama bin Laden (search), Myers said, "I can't tell you where he is," although he said the working assumption for months has been that he is hiding in the Afghan-Pakistan border area.

"There has been no diminution of our effort to go after" bin Laden, he said.