The U.S. military will pour thousands of troops into Afghanistan by next summer, and can expect to commit a sustained force for several more years, Defense Secretary Robert Gates and his top military commander there said Thursday.

Visiting Afghanistan for the second time in the past four months, where he is shown above, Gates got a short but intensive look at the leading military priority facing him and the incoming Obama administration in the coming months.

Just prior to his Kandahar meeting with Gen. David McKiernan, commander of U.S. and NATO troops in Afghanistan, Gates said the Pentagon is moving to get three of the four combat brigades requested by commanders into Afghanistan by late spring or early summer.

In his most specific comments to date about how soon he will meet the call for up to 20,000 more troops in Afghanistan, Gates said he will not have to cut troop levels further in Iraq to free up at least two of those three brigades for Afghan duty.

When the additions are complete, the number of U.S. troops in Afghanistan will climb to more than 50,000. Some 31,000 U.S. troops are there now.