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.
