guarantee

We are involved in an environmental project in which we give bank guarantees for the investors. We tried the only Islamic Institution (Amana Investments) in Sri Lanka to get the guarantees with 100% cash collateral, but very unfortunately they refused without any valid reason. Since we are in a non-Islamic country can we go into a state bank to issue guarantees as security for the investors? Because to get investors to this project we are forced to issue a bank guarantees as a security. Please do advice.