KANDAHAR, Afghanistan - The U.S. military will pour thousands of troops into Afghanistan by next summer and can expect to commit forces for several more years, Defense Secretary Robert M. Gates and his top military commander there said yesterday.

Just before meeting in Kandahar with Gen. David McKiernan, commander of U.S. and NATO troops in Afghanistan, Gates said the Pentagon was 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 would not have to cut troop levels further in Iraq to free 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, from the current 31,000.

Standing outside the military's headquarters for southern Afghanistan, Gates was surrounded by evidence of the coming buildup - swaths of land within the compound were crowded with construction equipment; skeletons of partially erected buildings stood nearby.

"This is a long fight, and I think we're in it until we are successful along with the Afghan people," Gates said. "I do believe there will be a requirement for sustained commitment here for some protracted period of time."

McKiernan added that it would be at least three to four years before Afghanistan could begin to build up its army and police enough so that they can operate more independently.

The general said a chunk of the new U.S. troops would go to the south, which has long been a Taliban stronghold and has seen recent spikes in violence as the insurgency reasserts itself.

Gates' visit comes as senior military leaders and the White House are pulling together a broad new military strategy, which would shift the focus from the waning fight in Iraq to the escalating Afghan battle.

Gates said troop levels in Iraq would remain fairly steady through the provincial elections early next year and "probably for some period of time after that."

McKiernan and Gates stressed that doubling the size of the Afghan army and increasing the ranks of the national police were critical.

The police, McKiernan said, will be the ones to maintain security in the cities and villages after the military moves in and clears out the insurgents.

Calling it a change that the administration of President-elect Barack Obama must tackle, Gates said the coalition forces must better cooperate with Kabul on security operations.

Officials have announced that the Third Brigade, 10th Mountain Division, would go to Afghanistan in January and that they would try to quickly meet the rest of the troop requirements.

Military leaders have resisted disclosing which other units would go, saying much depended on how soon troop levels could be cut in Iraq. A brigade is about 3,500 troops.