Dear Robert, a reference is Lazarsfeld, Positivity in Algebraic Geometry I, Corollary 2.1.38. Note that the statement there concerns the Iitaka dimension of any line bundle, not just the canonical bundle.
The basic idea of the proof is as follows: given a line bundle $L$ on $X$, after some birational modification (which doesn't change the Iitaka dimension of $L$), $X$ fibres over another variety $Y$ of dimension $\kappa_1(L)$ (in the OP's notation). One then compares the number of global sections of powers of $L$ with the number of global sections of some ample line bundles on $Y$, which one can compute with Riemann--Roch, to get the desired estimates.