US Secretary of Defense Ash Carter delivers remarks at a question-and-answer session with US military personnel at Kandahar Airfield in Kandahar February 22, 2015. The United States is considering slowing a planned withdrawal of US troops from Afghanistan to ensure that "progress sticks" after more than a decade of war, Carter said during an unannounced visit to Kabul on Saturday. [Photo/Agencies]

KANDAHAR, Afghanistan - US Defense Secretary Ash Carter said Sunday that training and advising Afghan security forces is "becoming the heart" of a US military mission that is winding down after 13 years of war.

Carter spoke to a gathering of about 100 soldiers, telling them that their work as advisers to the Afghan military is key to cementing security gains.

"We want it to be permanent and lasting," he said.

Kandahar is a regional hub for the US training and advising mission.

Carter told the soldiers that they deserve credit for improving the competence and professionalism of the Afghan army and police.

He said the Afghan security forces are strong and now "stand a chance" of prevailing over the Taliban in a lasting way.

Carter got briefed on American military operations at Kandahar airfield in southern Afghanistan as part of what he called a listening tour of the war zone in his first week as defense secretary.

On his second day in Afghanistan, Carter flew to Kandahar from Kabul, the capital, where he had held a series of talks with Afghan government leaders and American military commanders.

Kandahar is home to the Afghan Army's 205th Corps, elements of which are involved in a major offensive against the Taliban in neighboring Helmand province. US special operations forces also use the airfield at Kandahar to launch counterterrorism operations.