Starting from the effective action of high energy QCD we derive Ward identities for Green's functions of Reggeized gluons. They follow from the gauge invariance of the effective action, and allow us to derive new representations of amplitudes containing physical particles as well as Reggeized gluons. We explicitly demonstrate their validity for the Balitsky-Fadin-Kuraev-Lipatov kernel, and we present a new derivation of the kernel.