BANDA ACEH, Indonesia — An extremist Islamic group with alleged links to al Qaeda has set up a relief camp in the tsunami-stricken Aceh province on Sumatra island, raising concerns its fiercely anti-American members could stir up sentiment against U.S. and Australian troops helping to distribute aid. The group, Laskar Mujahidin, posted an English-language sign at the camp that reads, "Islamic Law Enforcement." The presence of the group, known for killing Christians during a long-running sectarian conflict in another part of Indonesia, generated fears that U.S. military personnel and others involved in relief work could become a terror target....