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

Gates visit comes at the same time U.S. President Barack Obama considers the scale and pace of troop withdrawals planned to start next month.

Shortly before the defense secretary's arrival in Kandahar Sunday, NATO announced one of its service members was killed in an insurgent attack in southern Afghanistan. In a separate incident, NATO said a coalition helicopter crashed in eastern Afghanistan, killing two on board.

As Gates arrived in the capital, Kabul, Saturday to begin his visit to the country, the international coalition announced that four service members were killed in an explosion in eastern Afghanistan.

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

Earlier Saturday at a security conference in Singapore, Gates said there could be peace talks with the Taliban within a year. Gates said the talks would be a result of NATO's ground advances in Afghanistan that have put pressure on the insurgents.

It has been nearly 10 years since U.S.-led forces invaded Afghanistan to topple the Taliban government and go after al-Qaida leader Osama bin Laden.

U.S. special forces killed bin Laden last month during a raid in neighboring Pakistan. Meanwhile, the U.S.-Afghan relationship has become more tense, with Afghan President Karzai repeatedly criticizing the U.S. and NATO for killing civilians during attacks on militants.