For the purpose of paying the cost of construction of sewer, drainage, or water systems or mains, or any or all of such sewer, drainage, or water systems or mains, the mayor and city council shall have power to issue warrants in amounts not to exceed the total sum of the special assessments provided for in section 16-670, which such warrants shall bear interest at such rate as the mayor and city council shall order. When there are no funds immediately available for the payment thereof, such warrants shall be registered in the manner provided for the registration of other warrants, and called and paid whenever there are funds available for the purpose in the manner provided for the calling and paying of other warrants. For the purpose of paying such warrants and the interest thereon from the time of their registration until paid, the special assessments pursuant to section 16-670 shall be kept as they are paid and collected in a fund to be designated and known as the Sewer and Water Extension Fund into which all money levied for such improvements shall be paid as collected, and out of which all warrants issued for such purposes shall be paid.