Until the bonds are paid, or until there is a sum in the city treasury set aside for the purpose, sufficient to meet all sums coming due for the principal and interest on the bonds, the city governing body shall, at the time of fixing, and in the manner provided for the general city tax levy, levy and collect each year upon the taxable property situated in the district, and upon that property only, a tax sufficient to pay the interest on the bonds as it falls due, and also such part of the principal as will become due before the proceeds of a tax levied at the next general tax levy will be available. However, if the maturity of the indebtedness created by the issuance of the bonds, or any series of the bonds, is made to begin more than two years after date of the bonds or such series, the tax shall be levied and collected annually, sufficient to pay the interest on the indebtedness as it falls due, and also to constitute a sinking fund for the payment of the principal on or before maturity.