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Testifying before a Senate panel, he said the earliest date for forces available for deployment to Afghanistan will be spring or summer of 2009.
Therefore, the focus now should be increasing the size of the Afghan army, he added.
"Last year, Joint Chiefs of Staff Chairman Mike Mullen noted that in Afghanistan we do what we can -- while in Iraq, we do what we must," Gates said.
However, "with the positive developments in Iraq, the strategic flexibility provided by ongoing troop reductions there, and the prospect of further reductions next year, I think it is possible in the months to come to do militarily what we must in both countries," he said.
Last week, David McKiernan, the top U.S. commander in Afghanistan, asked for an additional three brigades, beyond the one brigade that President George W. Bush recently announced will deploy to Afghanistan within months.
Gates on Tuesday expressed some caution about adding too many troops in Afghanistan.
The number of U.S. troops in Afghanistan has risen from less than 21,000 two years ago to more than 31,000 today, and that of allied troops has increased comparably.
Gates also said he sees "no downside" to the creation of a Afghan-Pakistani-U.S. patrol on the troubled Afghanistan-Pakistan border.
On Monday, Afghan Defense Minister Abdul Rahim Wardak told reporters at the Pentagon that the three countries had been discussing such as patrol.