You file on Schedule C.  According to the IRS:

If you are a U.S. citizen working in the United States for a foreign government or international organization, you must report this compensation as wages on Form 1040 and pay self-employment tax on the compensation under the Self-Employment Contributions Act (SECA). Self-employment tax is computed on Schedule SE, Self-Employment Tax, and is also reported on Form 1040. Although you are subject to self-employment tax on this employee compensation, you are not "self-employed" for any other federal tax purposes. You may not claim deductions for expenses on Schedule C and are not qualified to establish a Simplified Employee Pension (SEP) Plan and there is no allowable deduction for contributions to any such plan.