The Internal Revenue Service provides detailed information for nonresident aliens and is your best source for tax related questions.
International Tax
Anyone receiving payments from a U.S. source, whether they are U.S. residents or internationals are subject to U.S. taxation. There are specific regulations and processes required for determining the tax withholdings for international persons. In certain situations, there could be a tax treaty that may exist to exempt an international person from taxes or there may be regulations that allow a reduction in taxes.
For more information, please visit the Visa Office Website.