U.S. Defense Secretary Robert Gates, who is retiring at the end of the month, visited southern Afghanistan Sunday to say goodbye to American troops fighting Taliban insurgents.

In his remarks during visits to bases in Kandahar and Helmand provinces, Gates said a soon-to-begin U.S. troop withdrawal from Afghanistan should start with support troops, not combat soldiers.

The defense secretary also warned Afghan President Hamid Karzai that his country needs to increase security, and that failure to do so would put the smooth withdrawal of U.S. forces at risk.

Field commanders at both bases said they have seen significant security gains, but the violence rages on. Since Gates' arrival in Afghanistan Saturday, five NATO service members have died in insurgent-related violence.

Secretary Gates is expected to visit more U.S. troops Monday.

Later in the week he travels to Brussels for a NATO security conference. One of the topics will be the situation in Afghanistan.

Saturday, the defense secretary hinted that he saw a possible end in sight to the nearly 10-year-old war, thanks to security gains in the past 18 months.

At a security conference in Singapore, Gates said there could be peace talks with the Taliban within a year. He cautioned, however, the talks would only occur if NATO's ground advances in Afghanistan continue to put pressure on the insurgents.