Unknown. The key thing is that, to be deductible as mortgage interest, the mortgage must be secured by the home (this is sometimes called "perfected"). Basically, it means the loan must be recorded as a lien against the property in the county clerk's office, or wherever property records are kept, so that the bank could foreclose and take the house if you stopped making payments. If the loan is secured, then you can deduct the interest even though the bank is overseas and does not issue a 1098 form. (Up to the usual limits, of course.) If the loan is not secured, it does not count as a mortgage for the interest deduction.
