  Aims. High angular resolution N-band imaging is used to discern the torus of
active galactic nuclei (AGN) from its environment in order to allow a
comparison of its mid-infrared properties to the expectations of the unified
scenario for AGN. Methods. We present VLT-VISIR images of 25 low-redshift AGN
of different Seyfert types, as well as N-band SEDs of 20 of them. In addition,
we compare our results for 19 of them to Spitzer IRS spectra. Results. We find
that at a resolution of ~ 0.35", all the nuclei of our observed sources are
point-like, except for 2 objects whose extension is likely of instrumental
origin. For 3 objects, however, we observed additional extended circumnuclear
emission, even though our observational strategy was not designed to detect it.
Comparison of the VISIR photometry and Spitzer spectrophotometry indicates that
the latter is affected by extended emission in at least 7 out of 19 objects and
the level of contamination is (0.20 ~ 0.85) * F_IRS. In particular, the 10 um
silicate emission feature seen in the Spitzer spectra of 6 type I AGN, possibly
1 type II AGN and 2 LINERs, also probably originates not solely in the torus
but also in extended regions. Conclusions. Our results generally agree with the
expectations from the unified scenario, while the relative weakness of the
silicate feature supports clumpy torus models. Our VISIR data indicate that,
for low-redshift AGN, a large fraction of Spitzer IRS spectra are contaminated
by extended emission close to the AGN.
