  Flavour $SU(3)$ symmetry of strong interactions and certain dynamical
assumptions have been used in a series of recent publications to extract weak
CKM phases from $B$-decays into $\{\pi\pi,\pi K, K\bar K\}$ final states. We
point out that irrespectively of $SU(3)$-breaking effects the presence of
QCD-penguin contributions with internal $u$- and $c$-quarks precludes a clean
determination of the angle $\beta$ in the unitarity triangle by using the
branching ratios only. This difficulty can be overcome by measuring in addition
the ratio $x_d/x_s$ of $B^0_d-\bar B^0_d$ to $B^0_s-\bar B^0_s$ mixings. The
measurement of the angle $\gamma$ is unaffected by these new contributions.
Some specific uncertainties related to $SU(3)$-breaking effects and electroweak
penguin contributions are briefly discussed.
