Here are the links to the conflicting guidance:
IRS guidance (see section under = Green Card Holders (Lawful Permanent Resident):
https://www.irs.gov/individuals/international-taxpayers/employees-of-a-foreign-government-or-interna...
World Bank guidance (see last statement on last page of PDF):
http://siteresources.worldbank.org/INTRMA/Resources/TaxSTCFAQs.pdf
