The same place as any other pension income.

Where to report the income is the simple question.

Foreign pension, especially for non-treaty countries, could be a can of worms.  Question would include whether the foreign pension constitutes a grantor trust or §402(b) employee's trust, both of which come with their regulatory reporting requirements subject to substantial noncompliance penalties, and have different income tax implications, whether there's any nonqualified deferral subject to §409A, how much basis the taxpayer has (depending largely on how those prior year contributions and accretions were accounted for for US tax purposes), which part of the distribution is general limitation vs passive income, etc.