WASHINGTON (Reuters) - US lawmakers told President Barack Obamas top advisers on Thursday not enough was being done to combat corruption in Afghanistan, singling out allegations against the Afghan Presidents brother, whom Defence Secretary Robert Gates acknowledged was a problem. In a second day of congressional hearings on Obamas decision to send 30,000 more troops to Afghanistan to stem a resurgent Taliban, lawmakers also raised doubts about whether the administration was focused on the larger threat posed by Al-Qaeda men across the border in Pakistan. Gates said the scourge of corruption in Afghanistan was costing US taxpayers billions of dollars, and he suggested it was now more lucrative to skim off the top of US and international aid contracts than to trade in opium. He vowed not to work with corruption-tainted Afghan officials. Democratic Senator Robert Menendez questioned why the US had invested so heavily in Afghan President Hamid Karzai given Augusts fraud-marred election and his inability to provide security and basic services to many of Afghanistans 28 million people. What has he presided over? Menendez asked. Hes presided over massive corruption, where, you know, anywhere between 20 per cent or 40 per cent seems to be the going rate of skimming off of the taxpayers money, Menendez said. In a second hearing, Representative Gene Taylor, also a Democrat, questioned how the administration could be serious about tackling corruption and drugs in Afghanistan without addressing the alleged role of Ahmed Wali Karzai, the presidents brother, in the drug trade. Im just not going to talk about that in open session. We are dealing with a sovereign government, Gates said. Taylor insisted, Someone from the State Department admitted before this committee that it was true. Gates responded: Well, we have problems with him. Theres no question about it. Obama has been vague about what specific steps are being taken to get Pakistan to root out Al-Qaeda leadership, as well as Afghan Taliban long suspected of having links with elements of Pakistani intelligence. Officials said the administration was wary of talking publicly because of sensitivities in Islamabad, whose leaders are wary of being cast as American puppets. It is not clear how an expanded military effort in Afghanistan addresses the problem of Taliban and Al-Qaeda safe havens across the border in Pakistan, said Dick Lugar, the ranking Republican on the Senate Foreign Relations Committee. Democratic Senator John Kerry, the Committees Chairman, said what happens in Pakistan, particularly near the Afghan border, will do more to determine the outcome in Afghanistan than any increase in troops or shift in strategy. Gates singled out the threat posed by Al-Qaeda leaders operating out of Pakistan with relative impunity. He said Al-Qaeda, although weakened, was providing operational support to a range of groups seeking to destabilize Pakistan, including the Taliban and Lashkar-e-Taiba. Gates said Al-Qaeda was providing Lashkar-e-Taiba with targeting information to help the group plot attacks in India, clearly with the idea of provoking a conflict between India and Pakistan that would destabilise Pakistan. Admiral Mike Mullen, the Chairman of the Joint Chiefs of Staff, said the situation was extraordinarily dangerous. Secretary of State Hillary Clinton said recent Pakistani military offensives against local Taliban groups in the lawless Swat and Waziristan regions were important but were far from sufficient. But she played down the prospects of immediate action by Pakistan, where anti-American sentiment runs strong. This is an argument that takes time, she told the committee. There is a great gulf of mistrust.