As you are a US resident for taxation purposes, your contribution portion in the refund will not be taxable. Your contribution portion is considered as the cost of buying the pension plan. But the amount of income earned on the contribution you made will be taxable in the US even though this is a foreign contribution. So let me give you an example to explain this, you received a refund of $13,500. Your contribution to the pension plan was $10,000. Then the difference of $3,500 is taxable income in the US as you received this income while residing in the US. Had you paid taxes on it in the foreign country, then on your US return, you could claim a foreign tax credit to avoid double taxation on the same income. Here is the IRS link to this information:
https://www.irs.gov/businesses/the-taxation-of-foreign-pension-and-annuity-distributions
