  We constrain a stochastic background of primordial magnetic fields (PMF) by
its contribution to the cosmic microwave background (CMB) anisotropy angular
power spectrum with the combination of WMAP 7 year and South Pole Telescope
(SPT) data. The contamination in the SPT data by unresolved point sources and
by the Sunyaev Zeldovich (SZ) effect due to galaxy clusters has been taken into
account as modelled by the SPT collaboration. With this combination of WMAP 7
yr and SPT data, we constrain the amplitude Gaussian smoothed over 1 Mpc scale
of a stochatic background of non-helical PMF to $B_{\rm 1 Mpc}<3.5$ nG at 95%
confidence level. Our analysis shows that SPT data up to $\ell=3000$ bring an
improvement of almost a factor two with respect to results with previous CMB
high-$\ell$ data. We then discuss the forecasted impact from unresolved points
sources and SZ effect for {\sc Planck} capabilities in constraining PMF.
