If you must file a Federal income tax return, you cannot claim deductions and/or credits against your income from Puerto Rico since this is not taxable.

The deductions not attributed to a specific income will have to be prorated between the taxable gross income from the United States and all other income (including income from P.R.). Examples of deductions not attributed to a particular income are: alimony, itemized deductions such as medical expenses, charitable contributions, real estate taxes and mortgage interest of your principal residence. However, all personal exemptions are allowed.

You are not allowed to claim credits against non taxable income from Puerto Rico. Therefore, to calculate the credit for the foreign tax paid, you must reduce your foreign tax paid or accrued in Puerto Rico by the amount of the exempt income from Puerto Rico.

Residents in Puerto Rico can not claim the credit against accrued income. To claim this credit you must maintain a principal residence in the United States for more than a tax year.