EN
We investigate the classical embedding $B_{p,θ}^{s} ⊂ B_{q,θ}^{s-n(1/p-1/q)}$. The sharp asymptotic behaviour as s → 1 of the operator norm of this embedding is found. In particular, our result yields a refinement of the Bourgain, Brezis and Mironescu theorem concerning an analogous problem for the Sobolev-type embedding. We also give a different, elementary proof of the latter theorem.