U.S. Defense Secretary Robert Gates flew to Afghanistan Saturday in an unannounced visit to say farewell to US troops and Afghan leaders. Gates, who is retiring at the end of the month, arrived in Kabul on a flight from Singapore, where he reaffirmed U.S. commitments to Asian security earlier in the day. Gates planned to meet with soldiers and Marines in eastern and southern Afghanistan. He also was holding talks with Afghan President Hamid Karzai, who has been sharply critical in recent days of NATO airstrikes that have inadvertently killed and wounded civilians. Aides said it was Gates' 12th trip to Afghanistan since he became secretary of defense in December 2006. Gates told reporters before arriving in Kabul that although the Afghan war has proved costly in blood and treasure budget concerns should not cut short the drive to succeed.