This is what IRS has to say on the issue.

https://www.irs.gov/businesses/the-taxation-of-foreign-pension-and-annuity-distributions

Foreign social security pensions

Absent application of a particular treaty provision, foreign social security pensions are generally taxed as if they were foreign pensions or foreign annuities. They are not eligible for exclusion from taxable income the way a U.S. social security pension might be unless a tax treaty provides for an exclusion.

Most income tax treaties have special rules for social security payments. Generally, U.S. treaties provide that social security payments are taxable by the country making the payments. However, a foreign social security payment may also be taxable in the United States if you are a U.S. citizen or resident, as a result of the saving clause. And remember, not all treaties have the same provisions for foreign social security pensions, so always refer to the specific treaty at issue.