Subject to the conditions of this section and to such terms and conditions as the Administrator determines to be necessary to carry out the purposes of this subchapter, the Administrator is authorized to guarantee, and to make commitments to guarantee, the principal and interest (including interest accruing between the date of default and the date of the payment in full of the guarantee) of any loan, obligation, or participation therein of any State, municipality, or intermunicipal or interstate agency issued directly and exclusively to the Federal Financing Bank to finance that part of the cost of any grant-eligible project for the construction of publicly owned treatment works not paid for with Federal financial assistance under this subchapter (other than this section), which project the Administrator has determined to be eligible for such financial assistance under this subchapter, including, but not limited to, projects eligible for reimbursement under section
1286 of this title.