The Obama administration is expected on Tuesday to announce a large increase in its pledge to the Global Fund to Fight AIDS, Tuberculosis and Malaria, and to call for reform of the organization. The pledge of $4 billion over the next three fiscal years to the Geneva-based organization comes as governments and donors around the world have slowed increases in spending to combat HIV/AIDS, with weaker economies straining budgets. The U.S.-the largest contributor by far to the Global Fund, with more than $5.1 billion donated since 2002-is pressing the organization to develop an "action agenda" with timelines and measurements, "so that all parties concerned ... can be held accountable," a senior administration official said.