In his surprise visit to Afghanistan, US Defense Secretary Robert Gates has acknowledged that American troops are facing a "tough fight" in the war-torn country. Gates flew into Afghanistan's eastern Kunar province, near the Pakistani border, on Tuesday and is scheduled to meet Afghan President Hamid Karzai on Wednesday. He told reporters that militants' reliance on hideouts across the border has proved to be a persistent frustration for US-led forces. "I think these guys being close to the border do face some special challenges, and have taken some serious losses," Gates said. "There is no question that they are in a tough fight," he further explained. The Pentagon chief also raised questions over Pakistan's role and the prospects for weakening militancy in its tribal areas.