28 USC 1607 - Counterclaims

In any action brought by a foreign state, or in which a foreign state intervenes, in a court of the United States or of a State, the foreign state shall not be accorded immunity with respect to any counterclaim
(a) for which a foreign state would not be entitled to immunity under section 1605 of this chapter had such claim been brought in a separate action against the foreign state; or
(b) arising out of the transaction or occurrence that is the subject matter of the claim of the foreign state; or
(c) to the extent that the counterclaim does not seek relief exceeding in amount or differing in kind from that sought by the foreign state.