Dexia, the Belgian-French financial group, aims to protect its triple A rating by putting $300m into its US bond insurance arm and stopping writing protection on complex structured debt, the company said on Wednesday.

The capital injection comes with other measures to support the unit, Financial Security Assurance, which is one of the US-based bond insurers, or monolines, that have been hit hard by the credit crunch and the tumbling values of mortgage-backed bonds and other complex debt.